Lighting device and image display unit and light guide provided with it

ABSTRACT

An illumination device according to the present invention includes a light source; and a lightguide element including an incidence surface for receiving light emitted from the light source and an outgoing surface from which the light incident from the incidence surface goes out. The lightguide element includes a polarization selection layer for causing light of a specific polarization direction (first polarized light), among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting second polarized light, polarized in a different direction from that of the first polarized light, into the first polarized light. The polarization selection layer reflects the first polarized light substantially only toward the outgoing surface.

TECHNICAL FIELD

The present invention relates to an illumination device, an imagedisplay apparatus including the same, and a lightguide element, and moreparticularly relates to an illumination device having a high lightutilization efficiency, an image display apparatus including the same,and a lightguide element.

BACKGROUND ART

Recently, liquid crystal displays have been widely used for, forexample, office automation devices such as word processors and personalcomputers, mobile information devices such as PDAs, and camera-equippedVTRs including liquid crystal monitors owing to their features of beingthin and consuming low power.

Unlike light emitting displays such as CRTs (cathode ray tubes), PDPs(plasma display panels) and EL (electroluminescence) devices, non-lightemitting displays represented by liquid crystal displays do not emitlight themselves but control a transmitted amount or a reflected amountof externally emitted light to display characters and images.

The above-mentioned liquid crystal displays are roughly classified intotransmission type liquid crystal displays and reflection type liquidcrystal displays.

Transmission type liquid crystal displays perform display using light ofan illumination device (a so-called backlight) provided behind a liquidcrystal panel, whereas reflection type liquid crystal displays performdisplay using ambient light. Some known reflection type liquid crystaldisplays include an illumination device for improving the displayquality for the case where a sufficient strength of light is notobtained. Such an illumination device is called a “front light”, asopposed to a “backlight” which is an illumination device of atransmission type liquid crystal display.

Most of the transmission type liquid crystal displays practically usedtoday include a pair of polarizers facing each other with a liquidcrystal cell interposed therebetween. Most of the reflection type liquidcrystal displays practically used today include a polarizer provided onthe viewer side of the liquid crystal cell. Therefore, in the case whereillumination light emitted from an illumination device (a backlight or afront light) is in a randomly polarized state, about 50% of theillumination light is absorbed by the polarizer before being incident onthe liquid crystal cell.

In order to reduce the amount of light absorbed by the polarizer andthus to improve the light utilization efficiency, illumination devicesfor selectively emitting light of a predetermined polarization directionhave been proposed.

For example, Japanese Laid-Open Patent Publication No. 9-5739 and Tanaseand five others, “A New Backlighting System with a Polarizer Light Pipefor Enhanced Light Output from LCDs”, SID 97 DIGEST, pp. 365-368,disclose an illumination device for emitting light of a specifiedpolarization direction, utilizing that the reflectance at an interfacebetween transparent mediums having different refractive indices fromeach other has polarization dependency. FIG. 43 and FIG. 44schematically show an illumination device 740 disclosed in JapaneseLaid-Open Patent Publication No. 9-5739 and a liquid crystal display 700including the illumination device 740 as a backlight, respectively.

The liquid crystal display 700 includes a transmission type liquidcrystal display panel 710 and the illumination device (backlight) 740provided on the rear side of the liquid crystal display panel 710.

The liquid crystal display panel 710 includes a pair of substrates 711and 712, a liquid crystal layer 713 provided between the pair ofsubstrates 711 and 712, and a pair of polarizers 715 a and 715 bprovided outside the pair of substrates 711 and 712. The liquid crystaldisplay panel 710 performs display by modulating light, emitted from theillumination device 740 and incident on the liquid crystal display panel710 via the polarizer 715 b, by the liquid crystal layer 713 and thuscontrolling the amount of light transmitted through the polarizer 715 a.

The illumination device 740 includes a light source 741, a lightguideelement 720, and a reflection film 742 provided so as to surround thelight source.

The lightguide element 720 includes a first side surface (incidencesurface) 720 a on the side of the light source 741, a second sidesurface 720 b facing the first side surface 720 a, an outgoing surface720 c from which light incident from the light source 741 goes out, anda counter surface 720 d facing the outgoing surface 720 c. A λ/4 plate(quarter-wave plate) 732 and a reflection plate 734 are provided in thevicinity of the second side surface of the lightguide element 720, and areflection plate 736 is provided in the vicinity of the counter surface720 d of the lightguide element 720.

The lightguide element 720 is formed of a lightguide plate 721 and alightguide sheet 723 attached to each other. The lightguide sheet 723 isformed of transparent amorphous layers 723 a and 723 b which havedifferent refractive indices from each other and are alternately stackedat a predetermined angle.

Light, which is emitted from the light source 741 and is incident on theinside of the lightguide element 720 via the incidence surface 720 a, ispropagated toward the second side surface 720 b while being totallyreflected between the outgoing surface 720 c and the counter surface 720d repeatedly. A part of the light propagated in the lightguide element720 is reflected by interfaces between the amorphous layers 723 a and723 b forming the lightguide sheet 723, and goes out from the outgoingsurface 720 c toward the liquid crystal display panel 710.

It is known that the reflectance is different in accordance with thepolarization direction at the interfaces between the amorphous layershaving different refractive indices from each other. Especially whenlight is incident on such an interface at a specific angle of incidencewhich is referred to as the “Brewster angle”, the reflectance of Ppolarized light is zero and only S polarized light is reflected.

Accordingly, by stacking the amorphous layers 723 a and 723 b formingthe lightguide sheet 723 so as to be at an angle closer to the Brewsterangle with respect to the outgoing surface 720 c of the lightguideelement 720, the reflectance of first polarized light vibrating in adirection perpendicular to the direction in which the amorphous layers723 a and 723 b are alternately repeated (vibrating in the directionvertical to the sheet of FIG. 44) can be made high and the reflectanceof second polarized light vibrating in a direction parallel to thedirection in which the amorphous layers 723 a and 723 b are alternatelyrepeated (vibrating in the direction parallel to the sheet of FIG. 44)can be made low. Thus, the illumination light going out from thelightguide element 720 can have polarization characteristics.

The λ/4 plate 732 and the reflection plate 734 located in the vicinityof the second side surface 720 b of the lightguide element 720 areprovided in order to realize the following: the polarization directionof light, which does not go out from the outgoing surface 720 c of thelightguide element 720 and reaches the second side surface 720 b, isrotated and such light is again incident on the inside of the lightguideelement 720, and thus the light utilization efficiency is improved. Thereflection plate 736 located in the vicinity of the counter surface 720d of the lightguide element 720 is provided in order to reflect theillumination light, reflected toward the lightguide element 720 by theliquid crystal display panel 710, back toward the liquid crystal displaypanel 710.

In the liquid crystal display 700, light of a specific polarizationdirection is selectively emitted from the illumination device 740 asdescribed above. Therefore, absorption of light by the polarizer 715 bof the liquid crystal display panel 710 can be suppressed, and thus thelight utilization efficiency is improved.

Japanese PCT National Phase Laid-Open Publication No. 10-508151,Japanese PCT National Phase Laid-Open Publication No. 2001-507483, S. M.P. Blom and two others, “Towards Polarized Light Emitting Back Lights:Micro-structured Anisotropic Layers”, Asia Display/IDW '01, pp. 525-528,and Henri J. B. Jagt and three others, “Micro-structured PolymericLinearly Polarized Light Emitting Lightguide for LCD Illumination”, SID02 DIGEST, pp. 1236-1239; disclose an illumination device for emittinglight of a specific polarization direction, utilizing that thereflectance at an interface between a material having an isotropicrefractive index and a material having an anisotropic refractive indexhas polarization dependency. FIGS. 45(a), (b) and 46 schematically showan illumination device 800 disclosed in the Asia Display/IDW 101, pp.525-528.

The illumination device 800 includes a light source 810, a lightguideelement 820, and a reflection film 812 provided so as to surround thelight source 810.

The lightguide element 820 includes a first side surface (incidencesurface) 820 a on the side of the light source 810, a second sidesurface 820 b facing the first side surface 820 a, an outgoing surface820 c from which light incident from the light source 810 goes out, anda counter surface 820 d facing the outgoing surface 820 c.

The lightguide element 820 is formed of an isotropic layer 821 formed ofa material having an isotropic refractive index and an anisotropic layer823 formed of a material having an anisotropic refractive index whichare stacked on each other. A surface of the isotropic layer 821 on theside of the anisotropic layer 823 has grooves having a V-shaped crosssection formed therein at a constant pitch, and a surface of theanisotropic layer 823 on the side of the isotropic layer 821 hasprojections engageable with the V-shaped grooves formed thereon. Thus,the cross section of the interface between the isotropic layer 821 andthe anisotropic layer 823 is wave-like. The anisotropic layer 823 isdesigned such that only a refractive index ne in a specific direction isdifferent from a refractive index n of the isotropic layer 821 and arefractive index no in the other directions is almost the same as therefractive index n of the isotropic layer 821.

Light, which is emitted from the light source 810 and is incident on theinside the lightguide element 820 via the incidence surface 820 a, ispropagated toward the second side surface 820 b while being totallyreflected between the outgoing surface 820 c and the counter surface 820d repeatedly. A part of the light propagated in the lightguide element820 is reflected by parts of the interface between the anisotropic layer823 and the isotropic layer 821, the parts being inclining with respectto the outgoing surface 820 c, and goes out from the outgoing surface820 c.

At the interface between the anisotropic layer 823 and the isotropiclayer 821, only first polarized light vibrating in a direction in whichthe refractive indices thereof are different from each other isreflected, and second polarized light vibrating in a direction in whichthe refractive indices thereof are almost the same is not reflected.Therefore, the illumination light going out from the lightguide element820 can have polarization characteristics.

In the liquid crystal display 800, light of a specific polarizationdirection is selectively emitted from the outgoing surface 820 c asdescribed above. Therefore, the light utilization efficiency can beimproved.

Japanese PCT National Phase Laid-Open Publication No. 10-508151 alsodiscloses an illumination device for emitting light of a specificpolarization direction, utilizing that the reflectance at an interfacebetween an isotropic layer and an anisotropic layer has polarizationdependency, like the illumination device 800 shown in FIGS. 45(a), (b)and 46. Japanese PCT National Phase Laid-Open Publication No. 10-508151further discloses that as shown in FIGS. 45(a) and 46, the lightutilization efficiency can be further improved by providing adepolarizing reflection plate 832 in the vicinity of the second sidesurface 820 b of the lightguide element 820. The depolarizing reflectionplate 832 depolarizes the second polarized light which is not reflectedat the interface between the anisotropic layer 823 and the isotropiclayer 821 and causes a part of such light to be incident again on thelightguide element 820 as first polarized light. Therefore, the secondpolarized light can be utilized as the illumination light.

Japanese Laid-Open Patent Publication No. 9-218407 discloses anillumination device for emitting light of a specific polarizationdirection, utilizing the polarization dependency of diffraction in anarranged grating formed at an interface between an isotropic layer (alayer formed of a material having an isotropic refractive index) and ananisotropic layer (a layer formed of a material having an anisotropicrefractive index). FIGS. 47(a), (b) and 48 schematically show anillumination device 900 disclosed in Japanese Laid-Open PatentPublication No. 9-218407.

The illumination device 900 includes a light source 910, a lightguideelement 920, and a reflection film 912 provided so as to surround thelight source 910.

The lightguide element 920 includes a first side surface (incidencesurface) 920 a on the side of the light source 910, a second sidesurface 920 b facing the first side surface 920 a, an outgoing surface920 c from which light incident from the light source 910 goes out, anda counter surface 920 d facing the outgoing surface 920 c.

The lightguide element 920 is formed of an isotropic layer 921 formed ofa material having an isotropic refractive index and an anisotropic layer923 formed of a material having an anisotropic refractive index whichare stacked on each other. The anisotropic layer 923 is designed suchthat only a refractive index ne in a specific direction is differentfrom a refractive index n of the isotropic layer 921 and a refractiveindex no in the other directions is almost the same as the refractiveindex n of the isotropic layer 921. An interface between the isotropiclayer 921 and the anisotropic layer 923 is rectangular wave-like, andthe interface between the isotropic layer 921 and the anisotropic layer923 acts as an arranged grating. A phase plate 932 and a reflectionplate 934 are provided on the side of the counter surface 920 d of thelightguide element 920.

Light, which is emitted from the light source 910 and is incident on theinside the lightguide element 920 via the incidence surface 920 a, ispropagated toward the second side surface 920 b while being totallyreflected between the outgoing surface 920 c and the counter surface 920d repeatedly. A part of the light propagated in the lightguide element920 is diffracted by the arranged grating formed at the interfacebetween the anisotropic layer 923 and the isotropic layer 921, and goesout from the outgoing surface 920 c.

At the interface between the anisotropic layer 923 and the isotropiclayer 921, only first polarized light vibrating in a direction in whichthe refractive indices thereof are different from each other isreflected, and second polarized light vibrating in a direction in whichthe refractive indices thereof are almost the same is not reflected.Therefore, the illumination light going out from the lightguide element920 can have polarization characteristics.

In the liquid crystal display 900, light of a specific polarizationdirection is selectively emitted from the outgoing surface 920 c asdescribed above. Therefore, the light utilization efficiency can beimproved.

Japanese Laid-Open Patent Publication No. 9-218407 also describes thatthe second polarized light which is not diffracted by the arrangedgrating is converted into the first polarized light by the anisotropiclayer 923 and the phase plate 932 while being propagated in thelightguide element 920 toward the second side surface 920 b, andtherefore the second polarized light also can be utilized as theillumination light.

However, the illumination devices described above have the followingproblems.

In the illumination device 740 shown in FIG. 43 and FIG. 44 and theillumination device 800 shown in FIGS. 45 and 46, the second polarizedlight which is not directly reflected by the interface between theamorphous layers 723 a and 723 b, or the interface between the isotropiclayer 821 and the anisotropic layer 823, is converted into the firstpolarized light by the λ/4 plate 732 and the reflection plate 734provided in the vicinity of the second side surface 720 b of thelightguide element 720, or the depolarizing reflection plate 832provided in the vicinity of the second side surface 820 b of thelightguide element 820.

A transparent resin such as polymethylmethacrylate or polycarbonategenerally used as a material of a lightguide element has slightbirefringence. In order to convert the second polarized light which hasreached the second side surface 720 a of the lightguide element 720, orthe second side surface 820 a of the lightguide element 820, into thefirst polarized light by the λ/4 plate 732 and the reflection plate 734,or the depolarizing reflection plate 832, the birefringence of thelightguide element 720 or 820 needs to be suppressed sufficiently lowfor the following reason. When the lightguide element 720 or 820 has alarge birefringence, the second polarized light propagated in thelightguide element 720 or 820 is partially depolarized and reaches thesecond side surface as the first polarized light. Such light isconverted into second polarized light by the λ/4 plate 732 and thereflection plate 734 or the depolarizing reflection plate 832. As aresult, such light is not propagated toward the outgoing surface 720 cor 820 c after being incident again on the lightguide element 720 or820.

Therefore, in the illumination devices 740 and 800, the lightguideelements 720 and 820 each need to be formed of a material having asufficiently small birefringence, which restricts the range of usablematerials.

Recently, liquid crystal displays have been remarkably reduced inthickness, to the extent that the lightguide element 720 or 820 may beabout 0.7 mm to 0.8 mm thick at the second side surface 720 b or 820 b.For the production-related reasons, it is very difficult to locate theλ/4 plate 732 and the reflection plate 734, or the depolarizingreflection plate 832, in the vicinity of such a second side surface 720b or 820 b of the lightguide element 720 or 820 with high precision.Considering that the liquid crystal displays will become thinner in thefuture, such a structure is not practical.

Regarding the illumination device 900 shown in FIG. 47 and FIG. 48,patent document 4 describes that the second polarized light is convertedinto the first polarized light by the anisotropic layer 923. However, itis theoretically impossible that the second polarized light is convertedinto the first polarized light by the birefringence of the anisotropiclayer 923, because the first polarized light and the second polarizedlight respectively correspond to ordinary light and extra ordinary lightfor the anisotropic layer 923 in the illumination device 900. Therefore,in the illumination device 900, the second polarized light must beconverted into the first polarized light only by the phase plate 932.

However, Japanese Laid-Open Patent Publication No. 9-218407 describes nopractical specifications of the phase plate 932, for example, theanisotropy of the refractive index, thickness, and the direction of theoptical axis (slow axis or fast axis). Japanese Laid-Open PatentPublication No. 9-218407 does not disclose any knowledge required forefficiently converting the second polarized light into the firstpolarized light.

In addition, in the illumination device 900, light is diffracted towardthe counter surface 920 d by the arranged grating formed at theinterface between the isotropic layer 921 and the anisotropic layer 923as well as toward the outgoing surface 920 c. Therefore an unnegligibleamount of light goes out from the counter surface 920 d, which decreasesthe light utilization efficiency. When the illumination device 900 isused as a front light, light goes out toward the viewer and thus thedisplay quality is deteriorated.

As described above, an illumination device capable of causing light froma light source to go out as light of a specific polarization directionsufficiently efficiently has not been developed.

The present invention, made in light of the above-described problems,has an object of providing an illumination device capable of causinglight from a light source to go out as light of a specific polarizationdirection sufficiently efficiently, an image display apparatus includingthe same, and a lightguide element.

DISCLOSURE OF INVENTION

A first illumination device according to the present invention includesa light source; and a lightguide element including an incidence surfacefor receiving light emitted from the light source and an outgoingsurface from which the light incident from the incidence surface goesout. The lightguide element includes a polarization selection layer forcausing light of a specific polarization direction, among the lightincident from the incidence surface, to selectively go out from theoutgoing surface, and a polarization conversion layer for convertinglight of a polarization direction, different from the specificpolarization direction, into the light of the specific polarizationdirection. The polarization selection layer reflects the light of thespecific polarization direction substantially only toward the outgoingsurface. By this, the above-described object is achieved.

The polarization selection layer may include a plurality of incliningdielectric films provided at a predetermined angle with respect to theoutgoing surface.

A second illumination device according to the present invention includesa light source; and a lightguide element including an incidence surfacefor receiving light emitted from the light source and an outgoingsurface from which the light incident from the incidence surface goesout. The lightguide element includes a polarization selection layer forcausing light of a specific polarization direction, among the lightincident from the incidence surface, to selectively go out from theoutgoing surface, and a polarization conversion layer for convertinglight of a polarization direction, different from the specificpolarization direction, into the light of the specific polarizationdirection. The polarization selection layer includes a plurality ofinclining dielectric films inclining with respect to the outgoingsurface, and the plurality of inclining dielectric films are arrangedincreasingly densely as becoming farther from the incidence surface. Bythis, the above-described object is achieved.

The lightguide element may include a first member having a main surfacewhich includes a plurality of inclining surfaces inclining with respectto the outgoing surface and a plurality of parallel surfaces generallyparallel to the outgoing surface, and a second member provided on themain surface of the first member for flattening the main surface. Theplurality of inclining dielectric films may be respectively formed onthe plurality of inclining surfaces of the main surface; and theplurality of parallel surfaces of the main surface may be arrangedincreasingly sparsely as becoming farther from the incidence surface.

The polarization selection layer may include a plurality of furtherdielectric films respectively formed on the plurality of parallelsurfaces of the main surface.

The polarization selection layer may be located in the vicinity of theoutgoing surface and closer to the outgoing surface than thepolarization conversion layer. In this case, the plurality of parallelsurfaces are preferably located closer to the outgoing surface than theplurality of inclining surfaces.

The lightguide element may further include a counter surface facing theoutgoing surface, and the polarization selection layer may be located inthe vicinity of the counter surface and closer to the counter surfacethan the polarization conversion layer. In this case, the plurality ofparallel surfaces are preferably located closer to the counter surfacethan the plurality of inclining surfaces.

The first member, for example, is a prism sheet including a plurality ofprisms arranged on the main surface.

The second member, for example, is a transparent resin layer formed of atransparent resin material.

The polarization conversion layer may be formed of a transparentmaterial having birefringence.

The polarization conversion layer may be an injection-molded transparentresin layer.

The polarization conversion layer may be a phase plate.

It is preferable that directions of a slow axis and a fast axis of thephase plate in a plane parallel to the outgoing surface do not match thespecific polarization direction.

A third illumination device according to the present invention includesa light source; and a lightguide element including an incidence surfacefor receiving light emitted from the light source and an outgoingsurface from which the light incident from the incidence surface goesout. The lightguide element includes a polarization selection layer forcausing light of a specific polarization direction, among the lightincident from the incidence surface, to selectively go out from theoutgoing surface, and a polarization conversion layer for convertinglight of a polarization direction, different from the specificpolarization direction, into the light of the specific polarizationdirection. The polarization conversion layer is an injection-moldedtransparent resin layer having birefringence. By this, theabove-described object is achieved.

A fourth illumination device according to the present invention includesa light source; and a lightguide element including an incidence surfacefor receiving light emitted from the light source and an outgoingsurface from which the light incident from the incidence surface goesout. The lightguide element includes a polarization selection layer forcausing light of a specific polarization direction, among the lightincident from the incidence surface, to selectively go out from theoutgoing surface, and a polarization conversion layer for convertinglight of a polarization direction, different from the specificpolarization direction, into the light of the specific polarizationdirection. The polarization conversion layer is a phase plate.Directions of a slow axis and a fast axis of the phase plate in a planeparallel to the outgoing surface do not match the specific polarizationdirection. By this, the above-described object is achieved.

The phase plate may have monoaxial refractive index anisotropy.

In the case where the phase plate has monoaxial refractive indexanisotropy, it is preferable that a refractive index n_(x) in thedirection of the slow axis of the phase plate, a refractive index n_(y)in the direction of the fast axis of the phase plate, a refractive indexn_(z) in a thickness direction of the phase plate, a thickness d of thephase plate, a wavelength λ of visible light, and an angle α made by thespecific polarization direction and the slow axis of the phase platefulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈0,0<(n_(x)−n_(y))·d<λ, and 10°<α<30° or 40°<α<60°. It is more preferablethat the refractive index n_(x) in the direction of the slow axis of thephase plate, the refractive index n_(y) in the direction of the fastaxis of the phase plate, the refractive index n_(z) in the thicknessdirection of the phase plate, the thickness d of the phase plate, thewavelength λ of visible light, and the angle α made by the specificpolarization direction and the slow axis of the phase plate fulfill therelationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈0, (n_(x)−n_(y))·d=λ/2, and10°<α<30°.

Alternatively, it is preferable that a refractive index n_(x) in thedirection of the slow axis of the phase plate, a refractive index n_(y)in the direction of the fast axis of the phase plate, a refractive indexn_(z) in a thickness direction of the phase plate, a thickness d of thephase plate, a wavelength λ of visible light, and an angle α made by thespecific polarization direction and the slow axis of the phase platefulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈1,λ/4<(n_(x)−n_(y))·d<5λ/4, and 20°<α<90°. It is more preferable that therefractive index n_(x) in the direction of the slow axis of the phaseplate, the refractive index n_(y) in the direction of the fast axis ofthe phase plate, the refractive index n_(z) in the thickness directionof the phase plate, the thickness d of the phase plate, the wavelength λof visible light, and the angle α made by the specific polarizationdirection and the slow axis of the phase plate fulfill the relationshipof (n_(x)−n_(z))/(n_(x)−n_(y))≈1, (n_(x)−n_(y))·d=λ/2, and 20°<α<80°.

The phase plate may have biaxial refractive index anisotropy.

In the case where the phase plate has biaxial refractive indexanisotropy, it is preferable that a refractive index n_(x) in thedirection of the slow axis of the phase plate, a refractive index n_(y)in the direction of the fast axis of the phase plate, a refractive indexn_(z) in a thickness direction of the phase plate, a thickness d of thephase plate, a wavelength λ of visible light, and an angle α made by thespecific polarization direction and the slow axis of the phase platefulfill the relationship of 0.6<(n_(x)−n_(z))/(n_(x)−n_(y))<0.9,λ/4<(n_(x)−n_(y))·d<3λ/4, and 60°<α<80°. It is more preferable that therefractive index n_(x) in the direction of the slow axis of the phaseplate, the refractive index n_(y) in the direction of the fast axis ofthe phase plate, the refractive index n_(z) in the thickness directionof the phase plate, the thickness d of the phase plate, the wavelength λof visible light, and the angle α made by the specific polarizationdirection and the slow axis of the phase plate fulfill the relationshipof 0.6<(n_(x)−n_(z))/(n_(x)−n_(y))<0.9, (n_(x)−n_(y))·d=λ/2, and60°<α<80°.

The polarization conversion layer may be located oppositely to theoutgoing surface with the polarization selection layer interposedtherebetween.

The polarization conversion layer may be located closer to the outgoingsurface than the polarization selection layer.

An image display apparatus according to the present invention includesan illumination device having the above-described structure; and adisplay panel provided on the outgoing surface side of the lightguideelement of the illumination device and including at least one polarizer.By this, the above-described object is achieved.

The illumination device may further include a transparent input deviceformed on the counter surface of the lightguide element.

The display panel may include a substrate; and the lightguide elementincluded in the illumination device may act as the substrate.

A first lightguide element according to the present invention includesan incidence surface for receiving light emitted from a light source andan outgoing surface from which the light incident from the incidencesurface goes out. The lightguide element further includes a polarizationselection layer for causing light of a specific polarization direction,among the light incident from the incidence surface, to selectively goout from the outgoing surface, and a polarization conversion layer forconverting light of a polarization direction, different from thespecific polarization direction, into the light of the specificpolarization direction. The polarization selection layer reflects thelight of the specific polarization direction substantially only towardthe outgoing surface. By this, the above-described object is achieved.

A second lightguide element according to the present invention includesan incidence surface for receiving light emitted from a light source andan outgoing surface from which the light incident from the incidencesurface goes out. The lightguide element further includes a polarizationselection layer for causing light of a specific polarization direction,among the light incident from the incidence surface, to selectively goout from the outgoing surface, and a polarization conversion layer forconverting light of a polarization direction, different from thespecific polarization direction, into the light of the specificpolarization direction. The polarization selection layer includes aplurality of inclining dielectric films inclining with respect to theoutgoing surface, and the plurality of inclining dielectric films arearranged increasingly densely as becoming farther from the incidencesurface. By this, the above-described object is achieved.

A third lightguide element according to the present invention includesan incidence surface for receiving light emitted from a light source andan outgoing surface from which the light incident from the incidencesurface goes out. The lightguide element further includes a polarizationselection layer for causing light of a specific polarization direction,among the light incident from the incidence surface, to selectively goout from the outgoing surface, and a polarization conversion layer forconverting light of a polarization direction, different from thespecific polarization direction, into the light of the specificpolarization direction. The polarization conversion layer is aninjection-molded transparent resin layer having birefringence. By this,the above-described object is achieved.

A fourth lightguide element according to the present invention includesan incidence surface for receiving light emitted from a light source andan outgoing surface from which the light incident from the incidencesurface goes out. The lightguide element further includes a polarizationselection layer for causing light of a specific polarization direction,among the light incident from the incidence surface, to selectively goout from the outgoing surface, and a polarization conversion layer forconverting light of a polarization direction, different from thespecific polarization direction, into the light of the specificpolarization direction. The polarization conversion layer is a phaseplate. Directions of a slow axis and a fast axis of the phase plate in aplane parallel to the outgoing surface do not match the specificpolarization direction. By this, the above-described object is achieved.

Hereinafter, the functions of the present invention will be described.

In the first illumination device according to the present invention, thelightguide element includes a polarization selection layer for causinglight of a specific polarization direction, among the light incidentfrom the incidence surface, to selectively go out from the outgoingsurface, and a polarization conversion layer for converting light of apolarization direction, different from the specific polarizationdirection, into the light of the specific polarization direction.Accordingly, the light incident on the inside of the lightguide elementfrom the light source via the incidence surface can be caused to go outefficiently as light of the specific polarization direction. Therefore,the light utilization efficiency is improved. In addition, thepolarization selection layer reflects the light of the specificpolarization direction substantially only toward the outgoing surface.Therefore, the reduction in the light utilization efficiency and thereduction in the display quality (contrast ratio), which are caused bylight reflection toward the counter surface (toward the viewer in afront light), can be suppressed.

Owing to a structure in which the polarization selection layer includesa plurality of inclining dielectric films provided at a predeterminedangle with respect to the outgoing surface, the polarization selectionlayer can reflect the light of the specific polarization directionsubstantially only toward the outgoing surface.

In the second illumination device according to the present invention,the lightguide element includes a polarization selection layer forcausing light of a specific polarization direction, among the lightincident from the incidence surface, to selectively go out from theoutgoing surface, and a polarization conversion layer for convertinglight of a polarization direction, different from the specificpolarization direction, into the light of the specific polarizationdirection. Accordingly, the light incident on the inside of thelightguide element from the light source via the incidence surface canbe caused to go out efficiently as light of the specific polarizationdirection. Therefore, the light utilization efficiency is improved. Thepolarization selection layer includes a plurality of incliningdielectric films with respect to the outgoing surface, and theseinclining dielectric films reflect the light of the specificpolarization direction toward the outgoing surface. In the secondillumination device according to the present invention, the plurality ofinclining dielectric films are arranged increasingly densely as becomingfarther from the incidence surface. Therefore, the uniformity of thestrength of the light going out from the outgoing surface is enhanced.

The second illumination device according to the present invention can beeasily produced by, for example, constructing the lightguide element soas to include a first member having a main surface which includes aplurality of inclining surfaces inclining with respect to the outgoingsurface and a plurality of parallel surfaces generally parallel to theoutgoing surface, and a second member provided on the main surface ofthe first member for flattening the main surface; forming the pluralityof inclining dielectric films respectively on the plurality of incliningsurfaces of the main surface; and arranging the plurality of parallelsurfaces of the main surface increasingly sparsely as becoming fartherfrom the incidence surface.

In a structure in which the polarization selection layer includes aplurality of further dielectric films (i.e., dielectric films which aregenerally parallel to the outgoing surface) respectively formed on theplurality of parallel surfaces of the main surface, it is preferable toadopt a structure in which the incidence of light on the polarizationconversion layer is not prevented by these parallel dielectric films.Namely, it is preferable to adopt a structure in which the incidence oflight on the inclining dielectric films is not prevented by the paralleldielectric films. Specifically, it is preferable to adopt the followingstructures.

First, in the case where the polarization selection layer is located inthe vicinity of the outgoing surface, it is preferable that thepolarization selection layer is located closer to the outgoing surfacethan the polarization conversion layer. In such a structure, theincidence of light on the polarization conversion layer is not preventedby the parallel dielectric films, and thus the conversion of thepolarization direction into the specific polarization direction can bepreferably performed. In this case, it is preferable that the parallelsurfaces of the first member are located closer to the outgoing surfacethan the inclining surfaces of the first member, i.e., it is preferablethat the parallel dielectric films are located closer to the outgoingsurface than the inclining dielectric films. In such a structure, thelight is not prevented by the parallel dielectric films from reachingthe inclining dielectric films, and thus the light can preferably go outfrom the outgoing surface.

In the case where the lightguide element further includes a countersurface facing the outgoing surface and the polarization selection layeris located in the vicinity of the counter surface, it is preferable thatthe polarization selection layer is located closer to the countersurface than the polarization conversion layer. In such a structure, theincidence of light on the polarization conversion layer is not preventedby the parallel dielectric films, and thus the conversion of thepolarization direction into the specific polarization direction can bepreferably performed. In this case, it is preferable that the parallelsurfaces of the first member are located closer to the counter surfacethan the inclining surfaces of the first member, i.e., it is preferablethat the parallel dielectric films are located closer to the countersurface than the inclining dielectric films. In such a structure, thelight is not prevented by the parallel dielectric films from reachingthe inclining dielectric films, and thus the light can preferably go outfrom the outgoing surface.

As the first member, for example, a prism sheet including a plurality ofprisms arranged on the main surface is usable. As the second member, forexample, a transparent resin layer formed of a transparent resinmaterial is usable.

The polarization conversion layer is typically formed of a transparentmaterial having birefringence.

It is preferable that the polarization conversion layer is aninjection-molded transparent resin layer. In this case, it is easy toprovide a structure in which the polarization conversion layer is thickand occupies a majority of the area of the lightguide element. Thisenables a large amount of light to be propagated in the light conversionlayer to convert the light into the light of the specific polarizationdirection efficiently. It is preferable that the polarization conversionlayer is a phase plate. In this case, the slow axis thereof is generallyuniform (the same) in a plane parallel to the outgoing surface.Accordingly, the efficiency at which the light is converted into thelight of the specific polarization direction is generally uniform in aplane parallel to the outgoing surface. Therefore, it is easy to providea design in which the light of the specific polarization direction goesout uniformly from the outgoing surface.

Owing to a structure in which the directions of a slow axis and a fastaxis of the phase plate in a plane parallel to the outgoing surface donot match the specific polarization direction, the phase platepreferably acts as a polarization conversion layer.

In the third illumination device according to the present invention, thelightguide element includes a polarization selection layer for causinglight of a specific polarization direction, among the light incidentfrom the incidence surface, to selectively go out from the outgoingsurface, and a polarization conversion layer for converting light of apolarization direction, different from the specific polarizationdirection, into the light of the specific polarization direction.Accordingly, the light incident on the inside of the lightguide elementfrom the light source via the incidence surface can be caused to go outefficiently as light of the specific polarization direction. Therefore,the light utilization efficiency is improved. In addition, thepolarization conversion layer is an injection-molded transparent resinlayer having birefringence. Therefore, it is easy to provide a structurein which the polarization conversion layer is thick and occupies amajority of the area of the lightguide element. This enables a largeamount of light to be propagated in the light conversion layer toconvert the light into the light of the specific polarization directionefficiently.

In the fourth illumination device according to the present invention,the lightguide element includes a polarization selection layer forcausing light of a specific polarization direction, among the lightincident from the incidence surface, to selectively go out from theoutgoing surface, and a polarization conversion layer for convertinglight of a polarization direction, different from the specificpolarization direction, into the light of the specific polarizationdirection. Accordingly, the light incident on the inside of thelightguide element from the light source via the incidence surface canbe caused to go out efficiently as light of the specific polarizationdirection. Therefore, the light utilization efficiency is improved. Inaddition, the polarization conversion layer is a phase plate. Therefore,the slow axis thereof is generally uniform (the same) in a planeparallel to the outgoing surface. Accordingly, the efficiency at whichthe light is converted into the light of the specific polarizationdirection is generally uniform in a plane parallel to the outgoingsurface. Therefore, it is easy to provide a design in which the light ofthe specific polarization direction goes out uniformly from the outgoingsurface. Furthermore, the directions of the slow axis and the fast axisof the phase plate in a plane parallel to the outgoing surface do notmatch the specific polarization direction. Therefore, the phase platepreferably acts as a polarization conversion layer.

A phase plate having monoaxial refractive index anisotropy may be used.

In the case where the phase plate is monoaxial, it is preferable that arefractive index n_(x) in the direction of the slow axis of the phaseplate, a refractive index n_(y) in the direction of the fast axis of thephase plate, a refractive index n_(z) in a thickness direction of thephase plate, a thickness d of the phase plate, a wavelength λ of visiblelight, and an angle α made by the specific polarization direction andthe slow axis of the phase plate fulfill the relationship of(n_(x)−n_(z))/(n_(x)−n_(y))≈0, 0<(n_(x)−n_(y))·d<λ, and 10°<α<30° or40°<α<60°. In this case, the conversion of the light into the light ofthe specific polarization direction can be efficiently performed. It isespecially preferable that the refractive index n_(x) in the directionof the slow axis of the phase plate, the refractive index n_(y) in thedirection of the fast axis of the phase plate, the refractive indexn_(z) in the thickness direction of the phase plate, the thickness d ofthe phase plate, the wavelength λ of visible light, and the angle α madeby the specific polarization direction and the slow axis of the phaseplate fulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈0,(n_(x)−n_(y))·d=λ/2, and 10°<α<30°. In this case, the conversionefficiency does not vary almost at all in the wavelength range of thevisible light regardless of the wavelength, and thus the occurrence ofcoloring is suppressed.

In the case where the phase plate is monoaxial, it is also preferablethat a refractive index n_(x) in the direction of the slow axis of thephase plate, a refractive index n_(y) in the direction of the fast axisof the phase plate, a refractive index n_(z) in a thickness direction ofthe phase plate, a thickness d of the phase plate, a wavelength λ ofvisible light, and an angle α made by the specific polarizationdirection and the slow axis of the phase plate fulfill the relationshipof (n_(x)−n_(z))/(n_(x)−n_(y))≈1, λ/4<(n_(x)−n_(y))·d<5λ/4, and20°<α<90°. In this case also, the conversion of the light into the lightof the specific polarization direction can be efficiently performed. Itis especially preferable that the refractive index n_(x) in thedirection of the slow axis of the phase plate, the refractive indexn_(y) in the direction of the fast axis of the phase plate, therefractive index n_(z) in the thickness direction of the phase plate,the thickness d of the phase plate, the wavelength λ of visible light,and the angle α made by the specific polarization direction and the slowaxis of the phase plate fulfill the relationship of(n_(x)−n_(z))/(n_(x)−n_(y))≈1, (n_(x)−n_(y))·d=λ/2, and 20°<α<80°. Inthis case, the conversion efficiency does not vary almost at all in thewavelength range of the visible light regardless of the wavelength, andthus the occurrence of coloring is suppressed.

A phase plate having biaxial refractive index anisotropy may be used. Inthe case where the phase plate is biaxial, it is preferable that arefractive index n_(z) in the direction of the slow axis of the phaseplate, a refractive index n_(y) in the direction of the fast axis of thephase plate, a refractive index n_(z) in a thickness direction of thephase plate, a thickness d of the phase plate, a wavelength λ of visiblelight, and an angle α made by the specific polarization direction andthe slow axis of the phase plate fulfill the relationship of0.6<(n_(x)−n_(z))/(n_(x)−n_(y))<0.9, λ/4<(n_(x)−n_(y))·d<3λ/4, and60°<α<80°. In this case, the conversion of the light into the light ofthe specific polarization direction can be efficiently performed in awide range of angles (in a wide range of angle of propagation). It isespecially preferable that the refractive index n_(x) in the directionof the slow axis of the phase plate, the refractive index n_(y) in thedirection of the fast axis of the phase plate, the refractive indexn_(z) in the thickness direction of the phase plate, the thickness d ofthe phase plate, the wavelength λ of visible light, and the angle α madeby the specific polarization direction and the slow axis of the phaseplate fulfill the relationship of 0.6<(n_(x)−n_(z))/(n_(x)−n_(y))<0.9,(n_(x)−n_(y))·d=λ/2, and 60°<α<80°. In this case, the conversionefficiency does not vary almost at all in the wavelength range of thevisible light regardless of the wavelength, and thus the occurrence ofcoloring is suppressed.

The polarization conversion layer may be located oppositely to theoutgoing surface with the polarization selection layer interposedtherebetween (i.e., on the side of the counter surface facing theoutgoing surface), or on the side of the outgoing surface.

However, in the case where the polarization conversion layer is aninjection-molded transparent resin layer, it is preferable that thepolarization conversion layer is located oppositely to the outgoingsurface with the polarization selection layer interposed therebetween(i.e., on the side of the counter surface). In the case where thepolarization conversion layer is an injection-molded transparent resinlayer, the slow axis of the polarization conversion layer is disperse ina plane parallel to the outgoing surface. In a structure in which thepolarization conversion layer is located oppositely to the outgoingsurface with the polarization selection layer interposed therebetween(i.e., on the side of the counter surface), the light of the specificpolarization direction directed toward the outgoing surface by thepolarization selection layer does not pass the polarization conversionlayer before going out from the outgoing surface and thus is notdepolarized by the polarization conversion layer.

In the case where the polarization conversion layer is a phase plate,the slow axis of the polarization conversion layer is generally uniformin a plane parallel to the outgoing surface. It is preferable that thepolarization conversion layer is located closer to the outgoing surfacethan the polarization selection layer. In such a structure, thepolarization state (for example, the polarization direction) of thelight of the specific polarization direction which is directed towardthe outgoing surface by the polarization selection layer can becontrolled by the polarization conversion layer (phase plate).

An illumination device according to the present invention is preferablyusable for an image display apparatus. An image display apparatus,including an illumination device according to the present invention anda display panel provided on the outgoing surface side of the lightguideelement of the illumination device and including at least one polarizer,has a high light utilization efficiency and can provide bright display.

In a structure in which such an image display apparatus includes atransparent input device (a so-called touch panel) formed on the countersurface of the lightguide element of the illumination device, the imagedisplay apparatus can be thinner than a structure in which thetransparent input device, the illumination device and the display panelare simply stacked. A lightguide element having a transparent inputdevice formed on a counter surface thereof is obtained by, for example,adding a polarization selection layer and a polarization conversionlayer to a known transparent input device.

In a structure in which the display panel includes a substrate in suchan image display apparatus, it is preferable that the lightguide elementincluded in the illumination device acts as the substrate. In this case,the image display apparatus can be thinner than a structure in which theillumination device and the display panel are simply stacked.

The first lightguide element according to the present invention includesa polarization selection layer for causing light of a specificpolarization direction, among the light incident from the incidencesurface, to selectively go out from the outgoing surface, and apolarization conversion layer for converting light of a polarizationdirection, different from the specific polarization direction, into thelight of the specific polarization direction. Accordingly, the lightincident on the inside of the lightguide element from the light sourcevia the incidence surface can be caused to go out efficiently as lightof the specific polarization direction. Therefore, the light utilizationefficiency is improved. In addition, the polarization selection layerreflects the light of the specific polarization direction substantiallyonly toward the outgoing surface. Therefore, the reduction in the lightutilization efficiency and the reduction in the display quality(contrast ratio), which are caused by light reflection toward thecounter surface (toward the viewer in a front light), can be suppressed.

The second lightguide element according to the present inventionincludes a polarization selection layer for causing light of a specificpolarization direction, among the light incident from the incidencesurface, to selectively go out from the outgoing surface, and apolarization conversion layer for converting light of a polarizationdirection, different from the specific polarization direction, into thelight of the specific polarization direction. Accordingly, the lightincident on the inside of the lightguide element from the light sourcevia the incidence surface can be caused to go out efficiently as lightof the specific polarization direction. Therefore, the light utilizationefficiency is improved. The polarization selection layer includes aplurality of inclining dielectric films inclining with respect to theoutgoing surface, and these inclining dielectric films reflect the lightof the specific polarization direction toward the outgoing surface. Inthe second lightguide element according to the present invention, theplurality of inclining dielectric films are arranged increasinglydensely as becoming farther from the incidence surface. Therefore, theuniformity of the strength of the light going out from the outgoingsurface is enhanced.

The third lightguide element according to the present invention includesa polarization selection layer for causing light of a specificpolarization direction, among the light incident from the incidencesurface, to selectively go out from the outgoing surface, and apolarization conversion layer for converting light of a polarizationdirection, different from the specific polarization direction, into thelight of the specific polarization direction. Accordingly, the lightincident on the inside of the lightguide element from the light sourcevia the incidence surface can be caused to go out efficiently as lightof the specific polarization direction. Therefore, the light utilizationefficiency is improved. In addition, the polarization conversion layeris an injection-molded transparent resin layer having birefringence.Therefore, it is easy to provide a structure in which the polarizationconversion layer is thick and occupies a majority of the area of thelightguide element. This enables a large amount of light to bepropagated in the light conversion layer to convert the light into thelight of the specific polarization direction efficiently.

The fourth lightguide element according to the present inventionincludes a polarization selection layer for causing light of a specificpolarization direction, among the light incident from the incidencesurface, to selectively go out from the outgoing surface, and apolarization conversion layer for converting light of a polarizationdirection, different from the specific polarization direction, into thelight of the specific polarization direction. Accordingly, the lightincident on the inside of the lightguide element from the light sourcevia the incidence surface can be caused to go out efficiently as lightof the specific polarization direction. Therefore, the light utilizationefficiency is improved. In addition, the polarization conversion layeris a phase plate. Therefore, the slow axis thereof is uniform (the same)in a plane parallel to the outgoing surface Accordingly, the efficiencyat which the light is converted into the light of the specificpolarization direction is generally uniform in a plane parallel to theoutgoing surface. Therefore, it is easy to provide a design in which thelight of the specific polarization direction goes out uniformly from theoutgoing surface. Furthermore, the directions of the slow axis and thefast axis of the phase plate in a plane parallel to the outgoing surfacedo not match the specific polarization direction. Therefore, the phaseplate preferably acts as a polarization conversion layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an illuminationdevice 120 in Embodiment 1 according to the present invention and aliquid crystal display (image display apparatus) 100 including the same.

FIG. 2 is a cross-sectional view schematically showing the manner inwhich light is propagated in a lightguide element 20 of the illuminationdevice 120.

FIGS. 3(a), 3(b) and 3(c) are step cross-sectional views schematicallyshowing production steps of the illumination device 120 in Embodiment 1.

FIG. 4 is a graph showing the relationship between the outgoing angle(°) of the light from an outgoing surface 20 c and the relativeluminance (arbitrary unit; a.u.) in the illumination device 120 inEmbodiment 1.

FIG. 5 is a cross-sectional view schematically showing an illuminationdevice 220 in Embodiment 2 according to the present invention and aliquid crystal display (image display apparatus) 200 including the same.

FIG. 6 is a cross-sectional view schematically showing the manner inwhich light is propagated in a lightguide element 20 of the illuminationdevice 220.

FIGS. 7(a), 7(b) and 7(c) are step cross-sectional views schematicallyshowing production steps of the illumination device 220 in Embodiment 2.

FIG. 8 is a graph showing the relationship between the outgoing angle(°) of the light from an outgoing surface 20 c and the relativeluminance (arbitrary unit; a.u.) in the illumination device 220 inEmbodiment 2.

FIG. 9 is an isometric view schematically showing the relationship, inthe illumination device 200 among the refractive index n_(x) in thedirection of the slow axis of a phase plate, the refractive index n_(y)in the direction of the fast axis of the phase plate, the refractiveindex n_(z) in the thickness direction of the phase plate, the thicknessd of the phase plate, and the angle α made by the polarization directionP of first polarized light and the slow axis of the phase plate.

FIG. 10(a) is a graph showing the relationship between the efficiency(ratio) at which second polarized light is converted into the firstpolarized light after passing through the phase plate twice and theangle (°) at which the light is propagated in the phase plate, in thecase where a λ/4 plate (Nz=0 and monoaxial) for the light of λ=550 nm isused; and FIG. 10(b) is a graph showing the relationship between theconversion efficiency (ratio) into the first polarized light and theangle (°) of propagation when the λ/4 plate is located to realize α=50°.

FIG. 11(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a λ/2 plate (Nz=0 and monoaxial) for thelight of λ=550 nm is used; and FIG. 11(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ/2 plate is located to realize α=20°.

FIG. 12(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a 3λ/4 plate (Nz=0 and monoaxial) for thelight of λ=550 nm is used; and FIG. 12(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the 3λ/4 plate is located to realize α=20°.

FIG. 13(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a λ plate (Nz=0 and monoaxial) for thelight of λ=550 nm is used; and FIG. 13(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ plate is located to realize α=40°.

FIG. 14 is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation when a λ/4 plate (Nz=1 and monoaxial) for the light of λ=550nm is used.

FIG. 15(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a λ/2 plate (Nz=1 and monoaxial) for thelight of λ=550 nm is used; and FIG. 15(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ/2 plate is located to realize α=70°.

FIG. 16(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a 3λ/4 plate (Nz=1 and monoaxial) for thelight of λ=550 nm is used; and FIG. 16(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the 3λ/4 plate is located to realize α=80°.

FIG. 17(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a λ plate (Nz=1 and monoaxial) for thelight of λ=550 nm is used; and FIG. 17(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ plate is located to realize α=80°.

FIG. 18(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a 5λ/4 plate (Nz=1 and monoaxial) for thelight of λ=550 nm is used; and FIG. 18(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the 5λ/4 plate is located to realize α=60°.

FIG. 19 is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation when a λ/4 plate (Nz=0.5 and biaxial) for the light of λ=550nm is used.

FIG. 20 is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation when a λ/2 plate (Nz=0.9 and biaxial) for the light of λ=550nm is used.

FIG. 21(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a λ/2 plate (Nz=0.8 and biaxial) for thelight of λ=550 nm is used; and FIG. 21(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ/2 plate is located to realize α=70°.

FIG. 22(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a λ/2 plate (Nz=0.7 and biaxial) for thelight of λ=550 nm is used; and FIG. 22(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ/2 plate is located to realize α=70°.

FIG. 23 is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation when a λ/2 plate (Nz=0.6 and biaxial) for the light of λ=550nm is used.

FIG. 24(a) is a graph showing the relationship between the conversionefficiency (ratio) into the first polarized light and the angle (°) ofpropagation, in the case where a 3λ/4 plate (Nz=0.2 and biaxial) for thelight of λ=550 nm is used; and FIG. 24(b) is a graph showing therelationship between the conversion efficiency (ratio) and the angle (°)of propagation when the λ/2 plate is located to realize α=20°.

FIG. 25 is a cross-sectional view schematically showing an illuminationdevice 320 in Embodiment 3 according to the present invention and aliquid crystal display (image display apparatus) 300 including the same.

FIG. 26 is a cross-sectional view schematically showing the manner inwhich light is propagated in a lightguide element 20 of the illuminationdevice 320.

FIG. 27 is a cross-sectional view schematically showing the manner inwhich light goes out from an outgoing surface 20 c in the lightguideelement 20 in which dielectric films 22 a are arranged increasinglydensely as becoming farther from an incidence surface 20 a.

FIG. 28 is a cross-sectional view schematically showing the manner inwhich light goes out from an outgoing surface 20 c in a lightguideelement 20 in which dielectric films 22 a are arranged uniformly.

FIGS. 29(a), 29(b) and 29(c) are step cross-sectional viewsschematically showing production steps of the illumination device 320 inEmbodiment 3.

FIGS. 30(a) and 30(b) are cross-sectional views showing the manner inwhich a polarization selection layer 22 and a polarization conversionlayer 24 are located.

FIG. 31 is a graph showing the relationship between the outgoing angle(°) of the light from an outgoing surface 20 c and the relativeluminance (arbitrary unit; a.u.) in the illumination device 320 inEmbodiment 3.

FIG. 32 is an isometric view schematically showing the relationship, inthe illumination device 320, among the refractive index n_(x) in thedirection of the slow axis of a phase plate, the refractive index n_(y)in the direction of the fast axis of the phase plate, the refractiveindex n_(z) in the thickness direction of the phase plate, the thicknessd of the phase plate, and the angle α made by the polarization directionP of first polarized light and the slow axis of the phase plate.

FIGS. 33(a) and 33(b) are cross-sectional views showing the manner inwhich a polarization selection layer 22 and a polarization conversionlayer 24 are located.

FIGS. 34(a) and 34(b) are cross-sectional views showing the manner inwhich inclining dielectric films 22 a and parallel dielectric films 22 bare located.

FIGS. 35(a) and 35(b) are cross-sectional views showing the manner inwhich inclining dielectric films 22 a and parallel dielectric films 22 bare located.

FIG. 36 is a cross-sectional view schematically showing an illuminationdevice 420 in Embodiment 4 according to the present invention and aliquid crystal display (image display apparatus) 400 including the same.

FIG. 37 is a graph showing the relationship between the outgoing angle(°) of the light from an outgoing surface 20 c and the relativeluminance (arbitrary unit; a.u.) in the illumination device 420 inEmbodiment 4.

FIG. 38 is an isometric view schematically showing the relationship, inthe illumination device 420, among the refractive index n_(x) in thedirection of the slow axis of a phase plate, the refractive index n_(y)in the direction of the fast axis of the phase plate, the refractiveindex n_(z) in the thickness direction of the phase plate, the thicknessd of the phase plate, and the angle α made by the polarization directionP of first polarized light and the slow axis of the phase plate.

FIG. 39 is a cross-sectional view schematically showing an illuminationdevice 520 in Embodiment 5 according to the present invention and aliquid crystal display (image display apparatus) 500 including the same.

FIGS. 40(a), 40(b) and 40(c) are step cross-sectional viewsschematically showing production steps of the illumination device 520 inEmbodiment 5.

FIG. 41 is a cross-sectional view schematically showing an illuminationdevice 620 in Embodiment 6 according to the present invention and aliquid crystal display (image display apparatus) 600 including the same.

FIGS. 42(a), 42(b) and 42(c) are step cross-sectional viewsschematically showing production steps of the illumination device 620 inEmbodiment 6.

FIG. 43 is a cross-sectional view schematically showing a conventionalillumination device 740 and a liquid crystal display 700 including thesame.

FIG. 44 is a cross-sectional view schematically showing the manner inwhich light is propagated in a lightguide element 720 of theillumination device 740.

FIG. 45(a) is a cross-sectional view schematically showing aconventional illumination device 800, and FIG. 45(b) is an enlarged viewof an area 45B surrounded by the dashed line in FIG. 45(a).

FIG. 46 is a cross-sectional view schematically showing the manner inwhich light is propagated in a lightguide element 820 of theillumination device 800.

FIG. 47(a) is a cross-sectional view schematically showing aconventional illumination device 900, and FIG. 47(b) is an enlarged viewof an area 47B surrounded by the dashed line in FIG. 47(a).

FIG. 48 is a cross-sectional view schematically showing the manner inwhich light is propagated in a lightguide element 920 of theillumination device 900.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. The present invention is not limited to thefollowing embodiments.

Embodiment 1

First, with reference to FIG. 1, a structure of an illumination device120 in an embodiment according to the present invention and a structureof a liquid crystal display (image display apparatus) 100 including thesame will be described.

As shown in FIG. 1, the liquid crystal display 100 is a reflection typeliquid crystal display including a reflection type liquid crystaldisplay panel 110 and the illumination device (front light) 120.

The reflection type liquid crystal display panel 110 is a knownreflection type liquid crystal display panel, and includes a pair ofsubstrates (for example, glass substrates) 111 and 112 and a liquidcrystal layer 113 provided therebetween in this embodiment. A reflectionelectrode 114 is provided on the liquid crystal layer 113 side of therear-side substrate 111. A transparent electrode (not shown) is providedon the liquid crystal layer 113 side of the viewer-side substrate 112. Apolarizer (typically, a polarization plate) 115 and a λ/4 plate(quarter-wave plate) 116 are provided on the viewer side of thesubstrate 112.

The illumination device 120 includes a light source 10 and a lightguideelement 20. Typically, a reflection member 12 is provided so as tosurround the light source 10. The reflection member 12 causes lightemitted from the light source 10 to be incident on the lightguideelement 20 efficiently.

The lightguide element 20 is generally parallelepiped rectangular inthis embodiment, and includes a first side surface (incidence surface)20 a for receiving light emitted from the light source 10, a second sidesurface 20 b facing the first side surface 20 a, a third side surfaceand a fourth side surface (neither is shown) located between the firstside surface 20 a and the second side surface 20 b and facing eachother, and an outgoing surface 20 c and a counter surface 20 d facingeach other via these four side surfaces interposed therebetween.

The lightguide element 20 includes a polarization selection layer 22 anda polarization conversion layer 24 each having a predetermined thicknessin a direction normal to the outgoing surface 20 c.

The polarization selection layer 22 causes light of a specific directionof light incident on the incidence surface (first side surface) 20 a(hereinafter, also referred to as “first polarized light” for the sakeof convenience) to selectively go out from the outgoing surface 20 c.

In this embodiment, the polarization selection layer 22 includes aplurality of dielectric films 22 a provided at a predetermined anglewith respect to the outgoing surface 20 c. The dielectric films 22 a arearranged at a predetermined pitch, and has a refractive index differentfrom that of the material surrounding the dielectric films 22 a. Thedielectric films 22 a are typically dielectric thin films each having athickness of about several ten nanometers to several hundred nanometers.

Since the refractive index of the dielectric films 22 a is differentfrom the refractive index of the material surrounding the dielectricfilms 22 a, the light reflectance has polarization dependency at aninterface between the dielectric films 22 a and the material surroundingthe dielectric films 22 a. More specifically, the reflectance of Spolarized light is high and the reflectance of P polarized light is low.Especially, for light incident at an angle close to the Brewster angle,the reflectance of the P polarized light is almost zero and only the Spolarized light is reflected. Accordingly, at the interface between thedielectric films 22 a and the material surrounding the dielectric films22 a, the reflectance of light vibrating in a direction perpendicular tothe direction in which the dielectric films 22 a are repeated (vibratingin the direction vertical to the sheet of FIG. 1), and the reflectanceof light vibrating in a different direction from the direction of thisspecific light (first polarized light) is low. Therefore, the firstpolarized light is selectively reflected toward the outgoing surface 20c, and the first polarized light selectively goes out from the outgoingsurface 20 c.

In the polarization selection layer 22 including a plurality ofdielectric films 22 a provided (inclining) at a predetermined angle withrespect to the outgoing surface 20 c as in this embodiment, thedielectric films 22 a reflect the first polarized light included in thelight, which is incident on the inside of the lightguide element 20 viathe incidence surface 20 a, substantially only toward the outgoingsurface 20 c and reflect almost no such light toward the counter surface20 d. As a result, the first polarized light goes out mainly from theoutgoing surface 20 c and does not go out from the counter surface 20 dalmost at all.

On the other hand, the polarization conversion layer 24 converts lightpolarized in a different direction from that of the first polarizedlight (for example, light perpendicular to the first polarized light;hereinafter, also referred to as “second polarized light” for the sakeof convenience) into the first polarized light.

In this embodiment, the polarization conversion layer 24 is formed of atransparent material having birefringence. More specifically, thepolarization conversion layer 24 is an injection-molded transparentresin layer.

An injection-molded transparent resin layer, i.e., a transparent resinlayer which is formed by injection molding, has a slow axis which is notuniform but is disperse in a plane parallel to the outgoing surface 20c. Therefore, the second polarized light, which is different from thefirst polarized light, is depolarized by the birefringence of thetransparent resin layer while being propagated in the polarizationconversion layer 24, and a part of the depolarized light is convertedinto the first polarized light.

With reference to FIG. 2, the manner in which the light is propagated inthe lightguide element 20 will be described. In FIG. 2, the doublecircles with black inner circles indicate that the polarizationdirection of the light is vertical to the sheet of FIG. 2, and dashedarrows indicate that the polarization direction of the light is parallelto the sheet of FIG. 2.

Light emitted from the light source 10 is incident on the inside of thelightguide element 20 via the first side surface 20 a and is propagatedtoward the second side surface 20 b. Among the light propagated towardthe second side surface 20 b, the first polarized light vibrating in adirection perpendicular to the direction in which the dielectric films22 a are repeated (in this embodiment, the dielectric films 22 a arerepeated in a direction normal to the incident surface 20 a) isreflected toward the outgoing surface 20 c by the polarization selectionlayer 22 and goes out from the outgoing surface 20 c. Among the lightpropagated toward the second side surface 20 b, the second polarizedlight polarized in a direction perpendicular to that of the firstpolarized light is converted into the first polarized light by thepolarization conversion layer 24, and then is reflected toward theoutgoing surface 20 c by the polarization selection layer 22 and goesout from the outgoing surface 20 c. In this embodiment, the polarizationselection layer 22 including a plurality of dielectric films 22 a isused. In actuality, light is incident on the interface between thedielectric films 22 a and the material surrounding the dielectric films22 a also at angle other than the Brewster angle. Therefore, precisely,light other than the first polarized light is reflected by theinterface, and again precisely, light other than the first polarizedlight goes out from the outgoing surface 20 c. Accordingly, theexpression “causing the first polarization to selectively go out fromthe outgoing surface” means “causing the light dominated by the firstpolarization to selectively go out from the outgoing surface” and doesnot necessarily mean “causing only the first polarization to selectivelygo out from the outgoing surface”.

In this embodiment, the polarizer 115 of the reflection type liquidcrystal display panel 110 is located such that a transmission axisthereof is generally parallel to the polarization direction of the firstpolarized light which goes out from the outgoing surface 20 c.Therefore, the first polarized light going out from the illuminationdevice 120 is incident on the liquid crystal layer 113 without beingabsorbed by the polarizer 115 almost at all. It is not absolutelynecessary that the polarization direction of the first polarized lightand the transmission axis of the polarizer 115 is parallel to eachother. A phase plate may be provided between the polarizer 115 and theoutgoing surface 20 c for matching the polarization direction of thefirst polarized light to the direction of the transmission axis of thepolarizer 115.

As described above, in the illumination device 120 according to thepresent invention, the lightguide element 20 includes the polarizationselection layer 22 for causing the first polarized light to selectivelygo out from the outgoing surface 20 c and the polarization conversionlayer 24 for converting the second polarized light, polarized in adifferent direction from that of the first polarized light, into thefirst polarized light. Consequently, the light incident on the inside ofthe lightguide element 20 via the incidence surface 20 a from the lightsource 10 can be caused to go out efficiently as light of a specificpolarization direction. Therefore, the light utilization efficiency isimproved.

In the illumination device 120 in this embodiment, the polarizationselection layer 22 reflects the first polarized light substantially onlytoward the outgoing surface 20 c. Therefore, the reduction in the lightutilization efficiency and the reduction in the display quality(contrast ratio), which are caused by light reflection toward thecounter surface 20 d (toward the viewer), can be suppressed.

In addition, in the illumination device 120 in this embodiment, thepolarization conversion layer 24 is an injection-molded transparentresin layer. Therefore, it is easy to provide a structure in which thepolarization conversion layer 24 is thick and occupies a majority of thearea of the lightguide element 20. This enables a large amount of lightto be propagated in the light conversion layer 24 to convert the secondpolarized light into the first polarized light efficiently.

The illumination device 120 in this embodiment can be produced, forexample, as follows.

First, as shown in FIG. 3(a), a prism sheet 25 having a thickness of 0.2mm is formed of isotropic polymethylmethacrylate having a refractiveindex of 1.49. The prism sheet 25 has a front surface 25 a having asawtooth-like cross section and a generally flat rear surface 25 b. Therear surface 25 b is to become the outgoing surface 20 c later. Thefront surface 25 a is formed of inclining areas 25 a 1 inclining withrespect to the rear surface 25 b and vertical areas 25 a 2 which aregenerally vertical to the rear surface 25 b. The inclining areas 25 a 1and the vertical areas 25 a 2 are alternately arranged.

Next, as shown in FIG. 3(b), ZrO₂ having a refractive index of 2.10 isvapor-deposited on the inclining areas 25 a 1 of the front surface 25 aof the prism sheet 25, thereby forming dielectric films (dielectric thinfilms) 22 a having a thickness of 75 nm.

Then, as shown in FIG. 3(c), the prism sheet 25 and a transparent resinsheet 26 having a thickness of 0.8 mm which is formed by injectionmolding using polymethylmethacrylate having a refractive index of 1.49are bonded to each other via a transparent adhesive 27 having arefractive index of 1.49. In this manner, the lightguide element 20including the polarization selection layer 22 and the polarizationconversion layer 24 is obtained.

After that, the light source (for example, a cathode ray tube) 10 islocated on the incidence surface 20 a side of the lightguide element 20,and the reflection member (for example, a reflection film) 12 is locatedso as to surround the light source 10. Thus, the illumination device 120shown in FIGS. 1 and 2 is completed.

FIG. 4 shows the relationship between the outgoing angle (°) of thelight from the outgoing surface 20 c and the relative luminance(arbitrary unit; a.u.) in the illumination device 120 produced in thismanner. For the purpose of comparison, FIG. 4 also shows the luminanceof an illumination device produced in the same manner as theillumination device 120 except that the transparent resin sheet isformed by extrusion molding.

As can be seen from FIG. 4, the illumination device 120 produced using atransparent resin sheet formed by injection molding provides a higherluminance of the outgoing light than the illumination device producedusing a transparent resin sheet formed by extrusion molding. The reasonis that the transparent resin sheet formed by extrusion molding does notconvert the second polarized light into the first polarized lightefficiently, whereas the transparent resin sheet 26 formed by injectionmolding converts the second polarized light into the first polarizedlight efficiently.

It is known that the magnitude of birefringence of transparent resinlayers formed using a transparent resin (for example, theabove-described transparent resin sheets) varies in accordance with themolding method. For example, “Development and Characteristics of theLatest Resins for Optical Uses and Designing and Molding Technologies ofHigh Precision Components” (published by Technical Information InstituteCo., Ltd.), page 8, describes that the magnitude of birefringencechanges from highest to lowest in the order of injection molding,extrusion molding, compression molding and casting molding.

Accordingly, use of injection molding allows the birefringence of thetransparent resin sheet to be sufficiently large. Therefore, by using atransparent resin layer formed by injection molding as the polarizationconversion layer 24, the second polarized light propagated in thelightguide element 20 can be efficiently converted into the firstpolarized light.

When the polarization conversion layer 24 has the slow axis which isdisperse in a plane parallel to the outgoing surface 20 c (for example,like an injection-molded transparent resin layer as used in thisembodiment), the polarization conversion layer 24 is preferably locatedcloser to the counter surface 20 d than the polarization selection layer22 as in this embodiment.

When the polarization conversion layer 24 is located closer to thecounter surface 20 d than the polarization selection layer 22, the firstpolarization light directed (reflected) toward the outgoing surface 20 cby the polarization selection layer 22 does not pass through thepolarization conversion layer 24 before going out from the outgoingsurface 20 c, and thus is not depolarized by the polarization conversionlayer 24.

In this embodiment, polymethylmethacrylate is used as the material ofthe lightguide element 20 (the material of the prism sheet 25 and thetransparent resin sheet 26). The present invention is not limited tothis, and various transparent materials including polycarbonate areusable.

In this embodiment, the polarization selection layer 24 includes aplurality of dielectric films 22 a. The present invention is not limitedto this, and any type of polarization selection layer which can causelight of a specific polarization direction to selectively go out fromthe outgoing surface 20 c is usable. For example, a polarizationselection layer including a plurality of dielectric multiple-layer filmsprovided at a predetermined angle with respect to the outgoing surface20 c may be used. In consideration of the improvement in the lightutilization efficiency and the display quality, it is preferable to usea polarization selection layer capable of reflecting light of a specificpolarization direction substantially only toward the outgoing surface 20c.

Embodiment 2

With reference to FIG. 5, a structure of an illumination device 220 inan embodiment according to the present invention and a structure of aliquid crystal display (image display apparatus) 200 including the samewill be described. In the figures referred to below, elements havingsubstantially the same functions to those of the elements in theillumination device 120 and the liquid crystal display 100 in Embodiment1 bear the identical reference numerals thereto, and descriptionsthereof will be partially omitted, for the sake of simplicity ofdescription.

As shown in FIG. 5, the liquid crystal display 200 is a reflection typeliquid crystal display including a reflection type liquid crystaldisplay panel 210 and the illumination device (front light) 220.

The reflection type liquid crystal display panel 210 is a knownreflection type liquid crystal display panel, and has, for example, thesame structure as that of the reflection type liquid crystal displaypanel 110 of the liquid crystal display 100 in Embodiment 1.

The illumination device 220 is different from the illumination device120 in Embodiment 1 in that the lightguide element 20 includes a phaseplate as the polarization conversion layer 24.

In the illumination device 120 shown in FIG. 1, the polarizationconversion layer 24 is an injection-molded transparent resin layer andthe slow axis thereof is not uniform but is disperse in a plane parallelto the outgoing surface 20 c.

By contrast, in the illumination device 220 in this embodiment, thepolarization conversion layer 24 is a so-called phase plate, and theslow axis thereof is generally uniform (the same) in a plane parallel tothe outgoing surface 20 c. The polarization conversion layer 24 as thephase plate is designed such that the directions of the slow axis andthe fast axis (typically, perpendicular to the slow axis) thereof do notmatch the polarization direction of the first polarized light. Thesecond polarized light, which is different from the first polarizedlight, is converted into the first polarized light by the birefringence(linear birefringence) of the polarization conversion layer 24. As thepolarization conversion layer 24 acting as the phase plate, a λ/2 plate(half-wave plate), for example, is usable. Needless to say, thepolarization conversion layer 24 is not limited to a λ/2 plate, andphase plates other than the λ/2 plate can be used as described later.

With reference to FIG. 6, the manner in which light is propagated in thelightguide element 20 will be described.

Light emitted from the light source 10 is incident on the inside of thelightguide element 20 via the first side surface 20 a and is propagatedtoward the second side surface 20 b. Among the light propagated towardthe second side surface 20 b, first polarized light vibrating in adirection perpendicular to the direction in which the dielectric films22 a are repeated (in this embodiment, the dielectric films 22 a arerepeated in a direction normal to the incident surface 20 a) isreflected toward the outgoing surface 20 c by the polarization selectionlayer 22 and goes out from the outgoing surface 20 c. Among the lightpropagated toward the second side surface 20 b, second polarized lightpolarized in a direction perpendicular to that of the first polarizedlight is converted into the first polarized light by the polarizationconversion layer 24, and then is reflected toward the outgoing surface20 c by the polarization selection layer 22 and goes out from theoutgoing surface 20 c.

As described above, in the illumination device 220 in this embodimentalso, the lightguide element 20 includes the polarization selectionlayer 22 for causing the first polarized light to selectively go outfrom the outgoing surface 20 c and the polarization conversion layer 24for converting the second polarized light, polarized in a differentdirection from that of the first polarized light, into the firstpolarized light. Consequently, the light incident on the inside of thelightguide element 20 via the incidence surface 20 a from the lightsource 10 can be caused to go out efficiently as light of a specificpolarization direction. Therefore, the light utilization efficiency isimproved.

In the illumination device 220 in this embodiment, the polarizationconversion layer 24 is a phase plate, and thus the slow axis thereof isgenerally uniform (the same) in a plane parallel to the outgoing surface20 c. Therefore, the efficiency at which the second polarized light isconverted into the first polarized light is generally uniform in a planeparallel to the outgoing surface 20 c. This provides an advantage thatit is easy to design the illumination device such that the firstpolarized light uniformly goes out from the outgoing surface 20 c.

The illumination device 220 in this embodiment can be produced, forexample, as follows.

First, as shown in FIG. 7(a), a prism sheet 25 having a thickness of 1.0mm is formed of isotropic polymethylmethacrylate having a refractiveindex of 1.49. The prism sheet 25 has a front surface 25 a having asawtooth-like cross section and a generally flat rear surface 25 b. Thefront surface 25 a is formed of inclining areas 25 a 1 inclining withrespect to the rear surface 25 b and vertical areas 25 a 2 which aregenerally vertical to the rear surface 25 b. The inclining areas 25 a 1and the vertical areas 25 a 2 are alternately arranged.

Next, as shown in FIG. 7(b), ZrO₂ having a refractive index of 2.10 isvapor-deposited on the inclining areas 25 a 1 of the front surface 25 aof the prism sheet 25, thereby forming dielectric films (dielectric thinfilms) 22 a having a thickness of 75 nm.

Then, as shown in FIG. 7(c), the front surface 25 a of the prism sheet25 is flattened by a transparent resin 29 having a refractive index of1.49, and a λ/2 plate (produced by Nitto Denko Co., Ltd.) 28 formed ofARTON (registered trademark) having a refractive index of 1.51 is bondedto the rear surface 25 b of the prism sheet 25. In this manner, thelightguide element 20 including the polarization selection layer 22 andthe polarization conversion layer 24 is obtained.

After that, the light source (for example, a cathode ray tube) 10 islocated on the incidence surface 20 a side of the lightguide element 20,and a reflection member (for example, a reflection film) 12 is locatedso as to surround the light source 10. Thus, the illumination device 220shown in FIGS. 5 and 6 is completed.

FIG. 8 shows the relationship between the outgoing angle (°) of thelight from the outgoing surface 20 c and the relative luminance(arbitrary unit; a.u.) in the illumination device 220 produced in thismanner. FIG. 8 shows the luminance in the case where the followingrelationship is fulfilled by the following factors shown in FIG. 9: therefractive index n_(x) in the direction of the slow axis of the λ/2plate 28, the refractive index n_(y) in the direction of the fast axisof the λ/2 plate 28, the refractive index n_(z) in the thicknessdirection of the λ/2 plate 28, the thickness d of the λ/2 plate 28, thewavelength λ of the visible light (not shown), and the angle α made bythe polarization direction P of the first polarized light and the slowaxis of the λ/2 plate 28.(n _(x) −n _(y))·d=270 nm;(n _(x) −n _(z))/(n _(x) −n _(y))=1.0; andα=65°.

For the purpose of comparison, FIG. 8 also shows the luminance of anillumination device produced in the same manner as the illuminationdevice 220 except for not including a λ/2 plate (phase plate).

As can be seen from FIG. 8, the illumination device 220 including thepolarization conversion layer 24 as the phase plate provides a higherluminance of the outgoing light than the illumination device which doesnot include the phase plate, i.e., the polarization conversion layer. Inother words, it is appreciated that the second polarized light isefficiently converted into the first polarized light by the polarizationconversion layer 24 as the phase plate.

The specifications of the phase plate as the polarization conversionlayer 24 are not limited to those described herein as examples.Hereinafter, preferable specifications of the phase plate will bedescribed. Specifically, a preferable relationship among the refractiveindex n_(x) in the direction of the slow axis of the phase plate (theslow axis in a plane parallel to the outgoing surface 20 c), therefractive index n_(y) in the direction of the fast axis of the phaseplate (the fast axis in a plane parallel to the outgoing surface 20 c),the refractive index n_(z) in the thickness direction of the phaseplate, the thickness d of the phase plate, the wavelength λ of thevisible light, and the angle α made by the polarization direction P ofthe first polarized light and the slow axis of the phase plate.

First, a monoaxial phase plate of Nz=(n_(x)−n_(z))/(n_(x)−n_(y))=0,i.e., n_(x)=n_(z) will be described. FIGS. 10, 11, 12 and 13 show thecalculation results of the efficiency (ratio) at which the secondpolarized light is converted into the first polarized light afterpassing through such a phase plate having monoaxial refractive indexanisotropy twice. FIGS. 10(a), 11(a), 12(a) and 13(a) are graphs showingthe relationship between the conversion efficiency into the firstpolarized light and the angle (°) at which the light is propagated inthe phase plate, with the value of a being varied. FIGS. 10(b), 11(b),12(b) and 13(b) are graphs showing the relationship between theconversion efficiency into the first polarized light and the angle (°)at which the light is propagated in the phase plate, with the value of λbeing varied. The phase differences (n_(x)−n_(y))·d of the phase platesshown in FIGS. 10 through 13 are as shown in Table 1. TABLE 1 Nz (n_(x)− n_(y)) · d Remarks FIGS. 10(a), (b) 0 137.5 nm λ/4 plate for the lightof λ = 550 nm FIGS. 11(a), (b) 0 275.0 nm λ/2 plate for the light of λ =550 nm FIGS. 12(a), (b) 0 412.5 nm 3λ/4 plate for the light of λ = 550nm FIGS. 13(a), (b) 0 550.0 nm λ plate for the light of λ = 550 nm

The light propagated in the lightguide element 20 is totally reflectedin repetition between the outgoing surface 20 c and the counter surface20 d. Accordingly, the light propagated in the phase plate is at a totalreflection angle Oc or greater with respect to a plane parallel to theoutgoing surface 20 c. Therefore, the efficiency at which the secondpolarized light is converted into the first polarized light by the phaseplate is only needed to be considered for a range of the totalreflection angle θ or greater. When the lightguide element and the phaseplate are formed of generally used materials, i.e., transparent resinmaterials such as polymethylmethacrylate, polycarbonate and ARTON(registered trademark), the total reflection angle θc is about 40°.

FIGS. 10(a) and (b) are graphs showing the results obtained when a λ/4plate was used as the phase plate. FIG. 10(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 10(a), bylocating the λ/4 plate so as to realize, for example, α=50°, the secondpolarized light can be efficiently converted into the first polarizedlight. Especially, about 90% of the second polarized light propagated atan angle of about 60° in the λ/4 plate is converted into the firstpolarized light. FIG. 10(b) shows the conversion efficiency obtainedwhen α=50°. As can be seen from FIG. 10(b), the conversion efficiency ofthe second polarized light propagated at an angle of about 60° in theλ/4 plate is generally constant in the wavelength range of the visiblelight. This means that the occurrence of coloring of the first polarizedlight which goes out from the outgoing surface 20 c after the conversionis suppressed.

FIGS. 11(a) and (b) are graphs showing the results obtained when a λ/2plate was used as the phase plate. FIG. 11(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 11(a), bylocating the λ/2 plate so as to realize, for example, α=20°, the secondpolarized light can be efficiently converted into the first polarizedlight. Especially, about 90% of the second polarized light propagated atan angle of about 80° in the λ/2 plate is converted into the firstpolarized light. FIG. 11(b) shows the conversion efficiency obtainedwhen α=20°. As can be seen from FIG. 11(b), the conversion efficiency ofthe second polarized light propagated at an angle of about 80° in theλ/2 plate is generally constant in the wavelength range of the visiblelight.

FIGS. 12(a) and (b) are graphs showing the results obtained when a 3λ/4plate was used as the phase plate. FIG. 12(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 12(a), bylocating the 3λ/4 plate so as to realize, for example, α=20°, the secondpolarized light can be efficiently converted into the first polarizedlight. Especially, about 90% of the second polarized light propagated atan angle of about 45° in the 3λ/4 plate is converted into the firstpolarized light. FIG. 12(b) shows the conversion efficiency obtainedwhen α=20°. As can be seen from FIG. 12(b), the conversion efficiency ofthe second polarized light propagated at an angle of about 45° in the3λ/4 plate is generally constant in the wavelength range of the visiblelight.

FIGS. 13(a) and (b) are graphs showing the results obtained when a λplate was used as the phase plate. FIG. 13(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 13(a), bylocating the λ plate so as to realize, for example, α=10° or 40° through60°, the second polarized light can be efficiently converted into thefirst polarized light. The conversion efficiency is as high as 90% orgreater depending on the angle at which the light is propagated in the λplate. However, as can be seen from FIG. 13(b), when α=40° for example,the conversion efficiency into the first polarized light drasticallyvaries in the range of the visible light. As a result, the amount lightwhich goes out from the outgoing surface 20 c varies in accordance withthe wavelength, and coloring may occur.

The present inventor reviewed the above-described results in detail. Ina consequence, the following was found: when the phase plate hasmonoaxial refractive index anisotropy, the second polarized light can beefficiently converted into the first polarized light in the case wherethe refractive index n_(x) in the direction of the slow axis of thephase plate, the refractive index n_(y) in the direction of the fastaxis of the phase plate, the refractive index n_(z) in the thicknessdirection of the phase plate, the thickness d of the phase plate, thewavelength λ of the visible light, and the angle α made by thepolarization direction P of the first polarized light and the slow axisof the phase plate fulfill the following relationship (1).(n _(x) −n _(z))/(n _(x) −n _(y))≈0;0<(n _(x) −n _(y))·d<λ; and10°<α<30° or 40°<α<60°.  (1)

Especially when the following relationship (2) is fulfilled, theefficiency at which the second polarized light is converted into thefirst polarized light does not vary almost at all in the wavelengthrange of the visible light regardless of the wavelength, and thus theoccurrence of coloring is suppressed.(n _(x) −n _(z))/(n _(x) −n _(y))≈0;(n _(x) −n _(y))·d=λ/2; and10°<α<30°.  (2)

Next, a monoaxial phase plate of Nz=(n_(x)−n_(z))/(n_(x)−n_(y))=1, i.e.,n_(y)=n_(z) will be described. FIGS. 14, 15, 16, 17 and 18 show thecalculation results of the efficiency (ratio) at which the secondpolarized light is converted into the first polarized light afterpassing through such a phase plate having monoaxial refractive indexanisotropy twice. The phase differences (n_(x)−n_(y))·d of the phaseplates shown in FIGS. 14 through 18 are as shown in Table 2. TABLE 2 Nz(n_(x) − n_(y)) · d Remarks 1 137.5 nm λ/4 plate for the light of λ =550 nm FIGS. 15(a), (b) 1 275.0 nm λ/2 plate for the light of λ = 550 nmFIGS. 16(a), (b) 1 412.5 nm 3λ/4 plate for the light of λ = 550 nm FIGS.17(a), (b) 1 550.0 nm λ plate for the light of λ = 550 nm FIGS. 18(a),(b) 1 687.5 nm 5λ/4 plate for the light of λ = 550 nm

FIG. 14 is a graph showing the conversion efficiency of the light ofλ=550 nm obtained when a λ/4 was used as the phase plate. As can be seenfrom FIG. 14, use of the λ/4 plate allows the second polarized light tobe converted into the first polarized light, but the conversionefficiency is only about 70% at the maximum.

FIGS. 15(a) and (b) are graphs showing the results obtained when a λ/2plate was used as the phase plate. FIG. 15(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 15(a), bylocating the λ/2 plate so as to realize, for example, α=30° through 70°,the second polarized light can be efficiently converted into the firstpolarized light, and the conversion efficiency is as high as 90% orgreater depending on the angle of propagation of the light. FIG. 15(b)shows the conversion efficiency obtained when α=70°. As can be seen fromFIG. 15(b), the conversion efficiency does not vary almost at all in thewavelength range of the visible light regardless of the wavelength.

FIGS. 16(a) and (b) are graphs showing the results obtained when a 3λ/4plate was used as the phase plate. FIG. 16(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 16(a), bylocating the 3λ/4 plate so as to realize, for example, α=80°, the secondpolarized light can be efficiently converted into the first polarizedlight. Especially, 90% or greater of the second polarized lightpropagated at an angle of about 60° in the 3λ/4 plate is converted intothe first polarized light. FIG. 16(b) shows the conversion efficiencyobtained when α=80°. As can be seen from FIG. 16(b), the conversionefficiency of the second polarized light propagated at an angle of about60° in the 3λ/4 plate is generally constant in the wavelength range ofthe visible light.

FIGS. 17(a) and (b) are graphs showing the results obtained when a λplate was used as the phase plate. FIG. 17(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 17(a), bylocating the λ plate so as to realize, for example, α=40° through 50° or80°, the second polarized light can be efficiently converted into thefirst polarized light, and the conversion efficiency is as high as 90%or greater depending on the angle at which the light is propagated inthe λ plate. FIG. 17(b) shows the conversion efficiency obtained whenα=80°. As can be seen from FIG. 17(b), the conversion efficiency of thesecond polarized light propagated at an angle of about 65° in the λplate is generally constant in the wavelength range of the visiblelight.

FIGS. 18(a) and (b) are graphs showing the results obtained when a 5λ/4plate was used as the phase plate. FIG. 18(a) shows the conversionefficiency of the light of λ=550 nm. As can be seen from FIG. 18(a), bylocating the 5λ/4 plate so as to realize, for example, α=30° through60°, the second polarized light can be efficiently converted into thefirst polarized light, and the conversion efficiency is as high as 90%or greater depending on the angle at which the light is propagated inthe 5λ/4 plate. However, as can be seen from FIG. 18(b), when α=60° forexample, the conversion efficiency into the first polarized lightdrastically varies in the range of the visible light. As a result, theamount light which goes out from the outgoing surface 20 c varies inaccordance with the wavelength, and coloring may occur.

The present inventor reviewed the above-described results in detail. Ina consequence, the following was found: when the phase plate hasmonoaxial refractive index anisotropy, the second polarized light can beefficiently converted into the first polarized light also in the casewhere the refractive index n_(x) in the direction of the slow axis ofthe phase plate, the refractive index n_(y) in the direction of the fastaxis of the phase plate, the refractive index n_(z) in the thicknessdirection of the phase plate, the thickness d of the phase plate, thewavelength λ of the visible light, and the angle α made by thepolarization direction P of the first polarized light and the slow axisof the phase plate fulfill the following relationship (3).(n _(x) −n _(z))/(n _(x) −n _(y))≈1;λ/4<(n _(x) −n _(y))·d<5λ/4;20°<α<90°.  (1)

Especially when the following relationship (4) is fulfilled, theefficiency at which the second polarized light is converted into thefirst polarized light does not vary almost at all in the wavelengthrange of the visible light regardless of the wavelength, and thus theoccurrence of coloring is suppressed.(n _(x) −n _(z))/(n _(x) −n _(y))≈1;(n _(x) −n _(y))·d=λ/220°<α<80°.  (4)

So far, the monoaxial phase plates have been described. When a monoaxialphase plate is used, the range of angles of propagation at which theconversion efficiency is high may not be considered to be sufficientlywide as shown in FIGS. 10 through 18.

The present inventor found that use of a phase plate having biaxialrefractive index anisotropy enables the range of angles of propagationat which the conversion efficiency is high to be further widened. Thiswill be described below in more detail.

Specifically, a biaxial phase plate of0<Nz=(n_(x)−n_(z))/(n_(x)−n_(y))<1, i.e., n_(x)≠n_(z) and n_(y)≠n_(z)will be described. FIGS. 19, 20, 21, 22, 23 and 24 show the calculationresults of the efficiency (ratio) at which the second polarized light isconverted into the first polarized light after passing through such aphase plate having biaxial refractive index anisotropy twice. The phasedifferences (n_(x)−n_(y))·d of the phase plates shown in FIGS. 19through 24 are as shown in Table 3. TABLE 3 Nz (n_(x) − n_(y)) · dRemarks 0.5 137.5 nm λ/4 plate for the light of λ = 550 nm 0.9 275.0 nmλ/2 plate for the light of λ = 550 nm FIGS. 21(a), (b) 0.8 275.0 nm λ/2plate for the light of λ = 550 nm FIGS. 22(a), (b) 0.7 275.0 nm λ/2plate for the light of λ = 550 nm 0.6 275.0 nm λ/2 plate for the lightof λ = 550 nm FIGS 24(a), (b) 0.2 412.5 nm 3λ/4 plate for the light of λ= 550 nm

FIG. 19 is a graph showing the conversion efficiency of the light ofλ=550 nm obtained when a λ/4 of Nz=0.5 was used as the phase plate. Ascan be seen from FIG. 19, use of the λ/4 plate of Nz=0.5 allows thesecond polarized light to be converted into the first polarized light,but the conversion efficiency is only about 70% at the maximum.According to the review of the present inventor, when a λ/4 plate of0<Nz<1 was used, a conversion efficiency as high as 90% or greater couldnot be obtained in a wide range of angles of propagation.

FIG. 20 is a graph showing the conversion efficiency of the light ofλ=550 nm obtained when a λ/2 of Nz=0.9 was used as the phase plate. Ascan be seen from FIG. 20, use of the λ/2 plate of Nz=0.9 allows thesecond polarized light to be converted into the first polarized lightand the conversion efficiency is as high as 90% or greater depending onthe angle of propagation, but the range of angles of propagation atwhich the conversion efficiency is high cannot be considered to besufficiently wide.

FIGS. 21(a) and (b) are graphs showing the results obtained when a λ/2plate of Nz=0.8 was used as the phase plate. FIG. 21(a) shows theconversion efficiency of the light of λ=550 nm. As can be seen from FIG.21(a), by locating the λ/2 plate of Nz=0.8 so as to realize, forexample, α=70°, the second polarized light can be efficiently convertedinto the first polarized light, and the conversion efficiency is as highas 90% or greater in a wide range of angles of propagation of about 40°through 80°. FIG. 21(b) shows the conversion efficiency obtained whenα=70°. As can be seen from FIG. 21(b), the conversion efficiency,obtained when the λ/2 plate of Nz=0.8 is located so as to realize α=70°,is generally constant in the wavelength range of the visible light.

FIGS. 22(a) and (b) are graphs showing the results obtained when a λ/2plate of Nz=0.7 was used as the phase plate. FIG. 22(a) shows theconversion efficiency of the light of λ=550 nm. As can be seen from FIG.22(a), by locating the λ/2 plate of Nz=0.7 so as to realize, forexample, α=70°, the second polarized light can be efficiently convertedinto the first polarized light, and the conversion efficiency is as highas 90% or greater in a wide range of angles of propagation of about 40°through 70°. FIG. 22(b) shows the conversion efficiency obtained whenα=70°. As can be seen from FIG. 22(b), the conversion efficiency of thesecond polarized light, obtained when the λ/2 plate of Nz=0.7 is locatedso as to realize α=70°, is generally constant in the wavelength range ofthe visible light.

FIG. 23 is a graph showing the conversion efficiency of the light ofλ=550 nm obtained when a λ/2 of Nz=0.6 was used as the phase plate. Ascan be seen from FIG. 23, use of the λ/2 plate of Nz=0.6 allows thesecond polarized light to be converted into the first polarized light,and the conversion efficiency is as high as 90% or greater depending onthe angle of propagation, but the range of angles of propagation atwhich the conversion efficiency is high cannot be considered to besufficiently wide.

FIGS. 24(a) and (b) are graphs showing the results obtained when a 3λ/4plate of Nz=0.2 was used as the phase plate. FIG. 24(a) shows theconversion efficiency of the light of λ=550 nm. As can be seen from FIG.24(a), by locating the 3λ/4 plate of Nz=0.2 so as to realize, forexample, α=20°, the second polarized light can be efficiently convertedinto the first polarized light, and the conversion efficiency is as highas 90% or greater in a wide range of angles of propagation of about 50°through 70°. However, as can be seen from FIG. 24(b), in the case wherethe 3λ/4 plate of Nz=0.2 is used, when α=200 for example, the conversionefficiency into the first polarized light drastically varies in therange of the visible light. As a result, the amount light which goes outfrom the outgoing surface 20 c varies in accordance with the wavelength,and coloring may occur.

The present inventor reviewed the above-described results in detail. Ina consequence, the following was found: when the phase plate has biaxialrefractive index anisotropy, the second polarized light can beefficiently converted into the first polarized light in a wide range ofangles (a wide range of angles of propagation) in the case where therefractive index n_(x) in the direction of the slow axis of the phaseplate, the refractive index n_(y) in the direction of the fast axis ofthe phase plate, the refractive index n_(z) in the thickness directionof the phase plate, the thickness d of the phase plate, the wavelength λof the visible light, and the angle α made by the polarization directionP of the first polarized light and the slow axis of the phase platefulfill the following relationship (5).0.6<(n _(x) −n _(z))/(n _(x) −n _(y))<0.9;λ/4<(n _(x) −n _(y))·d<3λ/4;60°<α<80°.  (5)

Especially when the following relationship (6) is fulfilled, theefficiency at which the second polarized light is converted into thefirst polarized light does not vary almost at all in the wavelengthrange of the visible light regardless of the wavelength, and thus theoccurrence of coloring is suppressed.0.6<(n _(x) −n _(z))/(n _(x) −n _(y))<0.9;(n _(x) −n _(y))·d=λ/260°<α<80°.  (5)

In this embodiment, the polarization conversion layer 24 as the phaseplate is located closer to the counter surface 20 d than thepolarization selection layer 22. The present invention is not limited tothis, and the polarization conversion layer 24 may be located closer tothe outgoing surface 20 c than the polarization selection layer 22.

When the polarization conversion layer 24 is a phase plate as in thisembodiment, the slow axis of the polarization conversion layer 24 isgenerally the same in a plane parallel to the outgoing surface 20 c.Therefore, where the polarization conversion layer 24 (phase plate) islocated closer to the outgoing surface than the polarization selectionlayer 22, the polarization state (for example, the polarizationdirection) of the first polarized light which is directed toward theoutgoing surface 20 c by the polarization selection layer 22 can becontrolled by the phase plate, so that the first polarized light goesout from the outgoing surface 20 c.

Embodiment 3

With reference to FIG. 25, a structure of an illumination device 320 inan embodiment according to the present invention and a structure of aliquid crystal display (image display apparatus) 300 including the samewill be described.

As shown in FIG. 25, the liquid crystal display 300 is a reflection typeliquid crystal display including a reflection type liquid crystaldisplay panel 310 and the illumination device (front light) 320.

The reflection type liquid crystal display panel 310 is a knownreflection type liquid crystal display panel, and has, for example, thesame structure as that of the reflection type liquid crystal displaypanel 110 of the liquid crystal display 100 in Embodiment 1.

The lightguide element 20 of the illumination device 320 includes thepolarization selection layer 22 located in the vicinity of the outgoingsurface 20 c and the polarization conversion layer 24 located on thecounter surface 20 d side of the lightguide element 20. In thisembodiment, the polarization conversion layer 24 is a phase plate.

The polarization selection layer 22 includes dielectric films 22 ainclining at a predetermined angle with respect to the outgoing surface20 c (hereinafter, the dielectric films 22 a will also be referred to as“inclining dielectric films 22 a”) and dielectric films 22 b generallyparallel to the outgoing surface 20 c (hereinafter, the dielectric films22 b will also be referred to as “parallel dielectric films 22 b”).

The inclining dielectric films 22 a are arranged relatively sparsely inthe vicinity of the incidence surface (first side surface) 20 a, and arearranged relatively densely in the vicinity of the second side surface20 b. Namely, the inclining dielectric films 22 a are arrangedincreasingly densely as becoming farther from the incidence surface 20a.

By contrast, the parallel dielectric films 22 b are arranged relativelydensely in the vicinity of the incidence surface (first side surface) 20a, and are arranged relatively sparsely in the vicinity of the secondside surface 20 b. Namely, the parallel dielectric films 22 b arearranged increasingly sparsely as becoming farther from the incidencesurface 20 a.

The parallel dielectric films 22 b and the inclining dielectric films 22a are located in this order from the outgoing surface 20 c. Namely, theparallel dielectric films 22 b are located closer to the outgoingsurface 20 c than the inclining dielectric films 22 a. Accordingly, theinclining dielectric films 22 a and the parallel dielectric films 22 bincluded in the parallel selection layer 22, and the polarizationconversion layer 24 are located in the order of the parallel dielectricfilms 22 b, the inclining dielectric films 22 a and the polarizationconversion layer 24 from the outgoing surface 20 c.

With reference to FIG. 26, the manner in which light is propagated inthe lightguide element 20 will be described.

Light emitted from the light source 10 is incident on the inside of thelightguide element 20 via the first side surface 20 a and is propagatedtoward the second side surface 20 b. Among the light propagated towardthe second side surface 20 b, first polarized light vibrating in adirection perpendicular to the direction in which the dielectric films22 a are repeated (in this embodiment, the dielectric films 22 a arerepeated in a direction normal to the incident surface 20 a) isreflected toward the outgoing surface 20 c by the inclining dielectricfilms 22 a included in the polarization selection layer 22, and goes outfrom the outgoing surface 20 c.

Among the light propagated toward the second side surface 20 b, secondpolarized light polarized in a direction perpendicular to that of thefirst polarized light is converted into the first polarized light by thepolarization conversion layer 24, and then is reflected toward theoutgoing surface 20 c by the polarization selection layer 22 and goesout from the outgoing surface 20 c.

A part of the light propagated toward the second side surface 20 b isreflected toward the counter surface 20 d by the parallel dielectricfilms 22 b but is mostly incident on the counter surface 20 d at anangle equal to or greater than a critical angle (i.e., at an angle whichdoes not fulfill the total reflection condition). Therefore, such lightdoes not go out from the counter surface 20 d.

As described above, in the illumination device 320 in this embodimentalso, the lightguide element 20 includes the polarization selectionlayer 22 for causing the first polarized light to selectively go outfrom the outgoing surface 20 c and the polarization conversion layer 24for converting the second polarized light, polarized in a differentdirection from that of the first polarized light, into the firstpolarized light. Consequently, the light incident on the inside of thelightguide element 20 via the incidence surface 20 a from the lightsource 10 can be caused to go out efficiently as light of a specificpolarization direction. Therefore, the light utilization efficiency isimproved.

In the illumination device 320, the polarization conversion layer 24 isa phase plate, and thus the slow axis thereof is generally uniform (thesame) in a plane parallel to the outgoing surface 20 c. Therefore, theefficiency at which the second polarized light is converted into thefirst polarized light is generally uniform in a plane parallel to theoutgoing surface 20 c. This provides an advantage that it is easy todesign the illumination device such that the first polarized lightuniformly goes out from the outgoing surface 20 c.

Also in the illumination device 320, the inclining dielectric films 22 aincluded in the polarization selection layer 22 are arrangedincreasingly densely as becoming farther from the incidence surface 20 a(i.e., as becoming farther from the light source 10). As shown in FIG.27, this can further enhance the uniformity of the strength of the firstpolarized light going out from the outgoing surface 20 c.

By contrast, in the illumination device 220 shown in FIG. 5 (or in theillumination device 120 shown in FIG. 1), the dielectric films 22 ainclining at a predetermined angle with respect to the outgoing surface20 c are formed at a uniform rate regardless of the distance from theincidence surface 20 a. Therefore, as shown in FIG. 28, an excessiveamount of first polarized light may possibly go out in the vicinity ofthe incidence angle 20 a, and the amount of the first polarized lightmay possibly decrease as becoming farther from the incidence angle 20 a.This may reduce the uniformity of the strength of the light going outfrom the outgoing surface 20 c.

The illumination device 320 in this embodiment can be produced, forexample, as follows.

First, as shown in FIG. 29(a), a prism sheet 25 having a thickness of1.0 mm is formed of isotropic polymethylmethacrylate having a refractiveindex of 1.49. The prism sheet 25 has a main surface (front surface) 25a having a sawtooth-like cross section and a generally flat rear surface25 b. The main surface 25 a has a plurality of inclining surfaces(inclining areas) 25 a 1 inclining with respect to the rear surface 25b, a plurality of vertical surfaces (vertical areas) 25 a 2 which aregenerally vertical to the rear surface 25 b, and a plurality of parallelsurfaces (parallel areas) 25 a 3 which are generally parallel to therear surface 25 b. The plurality of inclining surfaces 25 a 1 arearranged increasingly densely as becoming farther from one end andcloser to the other end of the prism sheet 25 (as becoming farther fromthe side surface which will act as the incidence surface 20 a later).

Next, as shown in FIG. 29(b), TiO₂ having a refractive index of 2.3 isvapor-deposited on the inclining surfaces 25 a 1 of the main surface 25a of the prism sheet 25 to a thickness of 65 nm, thereby formingdielectric films (dielectric thin films) 22 a. At this stage, thedielectric films 22 b are also formed on the parallel surfaces 25 a 3 ofthe main surface 25 of the prism sheet 25. In FIG. 29(b), the arrowsschematically show the manner in which the dielectric material (TiO₂ inthis embodiment) is vapor-deposited.

Then, as shown in FIG. 29(c), the main surface 25 a of the prism sheet25 is flattened by a transparent resin 29 formed of a transparent resinmaterial having a refractive index of 1.49, and a monoaxial λ/2 plate(produced by Nitto Denko Co., Ltd.) 28 formed of ARTON (registeredtrademark) having a refractive index of 1.51 is bonded to the rearsurface 25 b of the prism sheet 25. In this manner, the lightguideelement 30 including the polarization selection layer 22 and thepolarization conversion layer 24 is obtained.

After that, the light source (for example, a cathode ray tube) 10 islocated on the incidence surface 20 a side of the lightguide element 20,and a reflection member (for example, a reflection film) 12 is locatedso as to surround the light source 10. Thus, the illumination device 320shown in FIGS. 25 and 26 is completed.

As described above, the illumination device 320 is obtained byconstructing the lightguide element 20 so as to include a first member(prism sheet 25) having the main surface 25 a which includes theplurality of inclining surfaces 25 a 1 and the plurality of parallelsurfaces 25 a 3 and include a second member (transparent resin layer 29)provided on the main surface 25 a for flattening the main surface 25 a,and by forming the dielectric films 22 a on the plurality of incliningsurfaces 25 a 1. By arranging the plurality of parallel surfaces 25 a 3increasingly sparsely as becoming farther from the incidence surface 20a, the plurality of inclining surfaces 25 a 1 can be arrangedincreasingly densely as becoming farther from the incidence surface 20a. Thus, a structure in which the inclining dielectric films 22 a arearranged increasingly densely as becoming farther from the incidencesurface 20 a can be easily realized. Even when a production process ofletting the dielectric films 22 b be also formed on the parallelsurfaces 25 a 3 is adopted, the light utilization efficiency and thedisplay quality are not deteriorated because the parallel dielectricfilms 22 b formed on the parallel surfaces 25 a 3 do not reflect thelight propagated in the lightguide element 20 toward the counter surface20 d at such an angle that the light goes out from the counter surface20 d.

In this embodiment, the polarization selection layer 22 is located inthe vicinity of the outgoing surface 20 c and the polarizationconversion layer 24 is located on the counter surface 20 d side of thelightguide element 20. The locations of the polarization selection layer22 and the polarization conversion layer 24 are not limited to this. Forexample, the polarization selection layer 22 may be located in thevicinity of the counter surface 20 d, and the polarization conversionlayer 24 may be located on the outgoing surface 20 c side of thelightguide element 20.

However, when the polarization selection layer 22 is located in thevicinity of the outgoing surface 20 c, it is preferable to locate, asshown in FIG. 30(b), the polarization selection layer 22 closer to theoutgoing surface 20 c than the polarization conversion layer 24, ratherthan locating, as shown in FIG. 30(a), the polarization conversion layer24 closer to the outgoing surface 20 c than the polarization selectionlayer 22.

The lightguide element 20 in this embodiment includes the dielectricfilms 22 b generally parallel to the outgoing surface 20 c in additionto the dielectric films 22 a inclining with respect to the outgoingsurface 20 c. Therefore, a part of the light propagated in thelightguide element 20 is incident on the parallel dielectric films 22 bat a large angle exceeding the Brewster angle, and the second polarizedlight is undesirably reflected by the parallel dielectric films 22 b.

Therefore, when the polarization conversion layer 24 is located closerto the outgoing surface 20 c than the polarization selection layer 22 asshown in FIG. 30(a), the second polarized light is unlikely to reach thepolarization conversion layer 24, which decreases the conversionefficiency into the first polarized light. By contrast, when thepolarization selection layer 22 is located closer to the outgoingsurface 20 c than the polarization conversion layer 24 as shown in FIG.30(b), the incidence of the second polarized light on the polarizationconversion layer 24 is not prevented by the parallel dielectric films 22b. As a result, the second polarized light can be preferably convertedinto the first polarized light.

FIG. 31 shows the relationship between the outgoing angle (°) of thelight from the outgoing surface 20 c and the relative luminance(arbitrary unit; a.u.) in the illumination device 320 in which the λ/2plate 28 as the polarization conversion layer 24 is located on thecounter surface 20 d side of the lightguide element 20 (the structureshown in FIG. 30(b)). FIG. 31 shows the luminance of the illuminationdevice 320 produced as described above with reference to FIG. 29. FIG.31 shows the luminance in the case where the following relationship isfulfilled by the following factors shown in FIG. 32: the refractiveindex n_(x) in the direction of the slow axis of the λ/2 plate 28, therefractive index n_(y) in the direction of the fast axis of the λ/2plate 28, the refractive index n_(z) in the thickness direction of theλ/2 plate 28, the thickness d of the λ/2 plate 28, the wavelength λ ofthe visible light (not shown), and the angle α made by the polarizationdirection P of the first polarized light and the slow axis of the λ/2plate 28.(n _(x) −n _(y))·d=270 nm;(n _(x) −n _(z))/(n _(x) −n _(y))=1.0; andα=70°.

For the purpose of comparison, FIG. 31 also shows the luminance of anillumination device in which the λ/2 plate (phase plate) 28 is locatedon the outgoing surface 20 c side of the lightguide element 20 (thestructure shown in FIG. 30(a)).

As can be seen from FIG. 31, the illumination device having thepolarization conversion layer 24 on the counter surface 20 d side of thelightguide element 20 provides a higher luminance of the outgoing lightthan the illumination device having the polarization conversion layer 24on the outgoing surface 20 c side of the lightguide element 20. In otherwords, the efficiency at which the second polarized light is convertedinto the first polarized light varies in accordance with the location ofthe polarization conversion layer 24. This is why when the polarizationselection layer 22 including the parallel dielectric films 22 b islocated in the vicinity of the outgoing surface 20 c, it is preferableto locate the polarization selection layer 22 closer to the outgoingsurface 20 c than the polarization conversion layer 24 as shown in FIG.30(b).

For the same reason, when the polarization selection layer 22 is locatedin the vicinity of the counter surface 20 c, it is preferable to locate,as shown in FIG. 33(b), the polarization selection layer 22 closer tothe counter surface 20 d than the polarization conversion layer 24,rather than locating, as shown in FIG. 33(a), the polarizationconversion layer 24 closer to the counter surface 20 d than thepolarization selection layer 22.

When the polarization conversion layer 24 is located closer to thecounter surface 20 d than the polarization selection layer 22 as shownin FIG. 33(a), the second polarized light is reflected by the paralleldielectric films 22 b and thus is unlikely to reach the polarizationconversion layer 24, which decreases the conversion efficiency into thefirst polarized light. By contrast, when the polarization selectionlayer 22 is located closer to the counter surface 20 d than thepolarization conversion layer 24 as shown in FIG. 33(b), the incidenceof the second polarized light on the polarization conversion layer 24 isnot prevented by the parallel dielectric films 22 b. As a result, thesecond polarized light can be preferably converted into the firstpolarized light.

When the polarization selection layer 22 is located in the vicinity ofthe outgoing surface 20 c and closer to the outgoing surface 20 c thanthe polarization conversion layer 24, it is preferable to locate, asshown in FIG. 34(b), the parallel dielectric films 22 b closer to theoutgoing surface 20 c than the inclining dielectric films 22 a, i.e., tolocate the parallel surfaces 25 a 3 of the prism sheet 25 closer to theoutgoing surface 20 c than the inclining surfaces 25 a 1, rather thanlocating, as shown in FIG. 34(a), the parallel dielectric films 22 bcloser to the counter surface 20 d than the inclining dielectric films22 a.

When the parallel dielectric films 22 b are located closer to thecounter surface 20 d than the inclining dielectric films 22 a as shownin FIG. 34(a), a part of the light propagated in the lightguide element20 is reflected by the parallel dielectric films 22 b and thus the lightis unlikely to reach the inclining dielectric films 22 a. As a result,the first polarized light is unlikely to go out from the outgoingsurface 20 c. By contrast, when the parallel dielectric films 22 b arelocated closer to the outgoing surface 20 c than the incliningdielectric films 22 a as shown in FIG. 34(b), the light propagated inthe lightguide element 20 reaches the inclining dielectric films 22 adirectly or after being reflected by the parallel dielectric films 22 b.Therefore, the light is not prevented by the parallel dielectric films22 b from reaching the inclining dielectric films 22 a. As a result, thefirst polarized light can preferably go out from the outgoing surface 20c.

For substantially the same reason, when the polarization selection layer22 is located in the vicinity of the counter surface 20 d and closer tothe counter surface 20 d than the polarization conversion layer 24, itis preferable to locate, as shown in FIG. 35(b), the parallel dielectricfilms 22 b closer to the counter surface 20 d than the incliningdielectric films 22 a, i.e., to locate the parallel surfaces 25 a 3 ofthe prism sheet 25 closer to the counter surface 20 d than the incliningsurfaces 25 a 1, rather than locating, as shown in FIG. 35(a), theparallel dielectric films 22 b closer to the outgoing surface 20 d thanthe inclining dielectric films 22 a.

When the parallel dielectric films 22 b are located closer to theoutgoing surface 20 c than the inclining dielectric films 22 a as shownin FIG. 35(a), a part of the light propagated in the lightguide element20 is reflected by the parallel dielectric films 22 b and thus the lightis unlikely to reach the inclining dielectric films 22 a. As a result,the first polarized light is unlikely to go out from the outgoingsurface 20 c. By contrast, when the parallel dielectric films 22 b arelocated closer to the counter surface 20 d than the inclining dielectricfilms 22 a as shown in FIG. 35(b), the light propagated in thelightguide element 20 reaches the inclining dielectric films 22 adirectly or after being reflected by the parallel dielectric films 22 b.Therefore, the light is not prevented by the parallel dielectric films22 b from reaching the inclining dielectric films 22 a. As a result, thefirst polarized light can preferably go out from the outgoing surface 20c.

In Embodiments 1 through 3 described above, the reflection type liquidcrystal displays 100, 200 and 300 respectively including theillumination devices 120, 220 and 320 as the front lights are described.The present invention is not limited to this, and is also preferablyapplicable to a transmission type liquid crystal display including anillumination device as a backlight.

Embodiment 4

With reference to FIG. 36, a structure of an illumination device 420 inan embodiment according to the present invention and a structure of aliquid crystal display (image display apparatus) 400 including the samewill be described.

As shown in FIG. 36, the liquid crystal display 400 is a transmissiontype liquid crystal display including a transmission type liquid crystaldisplay panel 410 and the illumination device (backlight) 420.

The transmission type liquid crystal display panel 410 is a knowntransmission type liquid crystal display panel, and includes a pair ofsubstrates (for example, glass substrates) 411 and 412 and a liquidcrystal layer 413 provided therebetween in this embodiment. Atransmission electrode (not shown) is provided on the liquid crystallayer 413 side of each of the substrates 411 and 412. Polarizers(typically, polarization plates) 415 a and 415 b are provided on theviewer side of the substrate 411 and the illumination device 420 side ofthe substrate 412, respectively.

The illumination device 420 has substantially the same structure as thatof the illumination device 320 shown in FIGS. 25 and 26, but isdifferent therefrom in that in the illumination device 420, thepolarization conversion layer 24 included in the lightguide element 20is a biaxial λ/2 plate. Namely, in the production process shown in FIG.29, a biaxial λ/2 plate 28 formed of ARTON (registered trademark) havinga refractive index of 1.51 is bonded to the rear surface 25 b of theprism sheet 25.

FIG. 37 shows the relationship between the outgoing angle (°) of thelight from the outgoing surface 20 c and the relative luminance(arbitrary unit; a.u.) in the illumination device 420. FIG. 37 shows theluminance in the case where the following relationship is fulfilled bythe following factors shown in FIG. 38: the refractive index n_(x) inthe direction of the slow axis of the biaxial λ/2 plate 28, therefractive index n_(y) in the direction of the fast axis of the biaxialλ/2 plate 28, the refractive index n_(x) in the thickness direction ofthe biaxial λ/2 plate 28, the thickness d of the biaxial λ/2 plate 28,the wavelength λ of the visible light (not shown), and the angle α madeby the polarization direction P of the first polarized light and theslow axis of the biaxial λ/2 plate 28.(n _(x) −n _(y))·d=270 nm;(n _(x) −n _(z))/(n _(x) −n _(y))=0.8; andα=70°.

For the purpose of comparison, FIG. 37 also shows the luminance of theillumination device 320 in Embodiment 3 including a monoaxial λ/2 plate(phase plate) as the polarization conversion layer 24.

As can be seen from FIG. 37, the illumination device 420 including abiaxial λ/2 plate as the polarization conversion layer 24 provides ahigher luminance of the outgoing light than the illumination device 320including the monoaxial λ/2 plate as the polarization conversion layer24. In other words, it is appreciated that the second polarized light ismore efficiently converted into the first polarized light by thepolarization conversion layer 24 which is a biaxial λ/2 plate.

In the illumination device 420 in this embodiment, a reflection member(for example, a reflection film) may be provided on the counter surface20 d side of the lightguide element 20, and a light scattering member(for example, a light scattering film) may be provided on the outgoingsurface 20 c side of the lightguide element 20.

In this embodiment, the transmission type liquid crystal display 400including the illumination device 420 as a backlight is described.Alternatively, the illumination device 420 may be used as a front lightof a reflection type liquid crystal display.

Embodiment 5

With reference to FIG. 39, a structure of an illumination device 520 inan embodiment according to the present invention and a structure of aliquid crystal display (image display apparatus) 500 including the samewill be described.

As shown in FIG. 39, the liquid crystal display 500 is a reflection typeliquid crystal display including a reflection type liquid crystaldisplay panel 510 and the illumination device (front light) 520.

The reflection type liquid crystal display panel 510 is a knownreflection type liquid crystal display panel, and has, for example, thesame structure as that of the reflection type liquid crystal displaypanel 110 of the liquid crystal display 100 in Embodiment 1.

The illumination device 520 is different from the above-describedillumination devices 120, 220, 320 and 420 in that in the illuminationdevice 520, a transparent input device (touch panel) 530 is provided onthe counter surface 20 d of a lightguide element 20A. In FIG. 39, thepolarization selection layer 22 and the polarization conversion layer 24included in the lightguide element 20A have the same structure as thatof the polarization selection layer 22 and the polarization conversionlayer 24 included in the lightguide element 20 of each of theillumination devices 320 and 420. Alternatively, the polarizationselection layer 22 and the polarization conversion layer 24 of thelightguide element 20A may have the same structure as that of thepolarization selection layer 22 and the polarization conversion layer 24of each of the illumination devices 120 and 220.

The touch panel 530 includes a lower electrode (typically, a transparentconductive film; not shown) and spacers 531 which are formed on thecounter surface 20 d of the lightguide element 20A, and an upperelectrode film 532. The upper electrode film 532 has an upper electrode(typically, a transparent conductive film; not shown) formed on asurface thereof on the side of the lightguide element 20A and is bondedto the counter surface 20 d of the lightguide element 20A by an adhesive531. In the transparent input device 530, the upper electrode and thelower electrode become conductive to each other in accordance with thedeformation which is caused by the upper electrode film 532 beingpressed, and thus information is input.

The illumination device 520 in this embodiment can be produced, forexample, as follows.

First, as shown in FIG. 40(a), prisms are formed of a transparent resin29 having a refractive index of 1.51 on a phase plate 28 formed of ARTON(registered trademark) having a refractive index of 1.51, and thus aprism sheet 25′ having a thickness of 0.2 mm is formed. The prism sheet25′ has a main surface (front surface) 25 a′ having a sawtooth-likecross section and a generally flat rear surface 25 b′. The main surface25 a′ has inclining surfaces (inclining areas) 25 a 1 inclining withrespect to the rear surface 25 b′, vertical surfaces (vertical areas) 25a 2′ which are generally vertical to the rear surface 25 b′, andparallel surfaces (parallel areas) 25 a 3′ which are generally parallelto the rear surface 25 b′. The inclining surfaces 25 a 1′ are arrangedincreasingly densely as becoming farther from one end and closer to theother end of the prism sheet 25′ (as becoming farther from the sidesurface which will act as the incidence surface 20 a later).

Next, as shown in FIG. 40(b), TiO₂ having a refractive index of 2.3 isvapor-deposited on the inclining surfaces 25 a 1′ of the main surface 25a′ of the prism sheet 25′ to a thickness of 65 nm, thereby formingdielectric films (dielectric thin films) 22 a. At this stage, dielectricfilms 22 b are also formed on the parallel surfaces 25 a 3′ of the mainsurface 25′.

Then, as shown in FIG. 40(c), the main surface 25 a′ of the prism sheet25′ is flattened by the transparent resin 29 having a refractive indexof 1.51, and a transparent substrate (for example, a glass substrate) 26having a thickness of 0.7 mm and having the above-described transparentinput device (touch panel) 530 formed thereon is bonded to the rearsurface 25 b′ of the prism sheet 25′.

After that, the light source (for example, a cathode ray tube) 10 islocated on the incidence surface 20 a side of the lightguide element20A, and a reflection member (for example, a reflection film) 12 islocated so as to surround the light source 10. Thus, the illuminationdevice 520 shown in FIG. 39 is completed.

In the reflection type illumination device 500 in this embodiment, thelightguide element 20A of the illumination device 520 as a front lightand the transparent input device 530 are integrated together. Therefore,the input function can be added without significantly increasing thethickness.

Embodiment 6

With reference to FIG. 41, a structure of an illumination device 620 inan embodiment according to the present invention and a structure of aliquid crystal display (image display apparatus) 600 including the samewill be described.

As shown in FIG. 41, the liquid crystal display 600 is a transmissiontype liquid crystal display including a transmission type liquid crystaldisplay panel 610 and the illumination device (backlight) 620.

The transmission type liquid crystal display panel 610 has substantiallythe same structure as that of the transmission type liquid crystaldisplay panel 410 of the liquid crystal display 400 in Embodiment 4. Thetransmission type liquid crystal display panel 610 is different from theabove-described transmission type liquid crystal display panel 410 inthat the transmission type liquid crystal display panel 610 includes thepolarization selection layer 22 and the polarization conversion layer 24formed on the substrate 412, instead of the polarizer.

The polarization selection layer 22 and the polarization conversionlayer 24 in this embodiment have substantially the same structure asthat of the polarization selection layer 22 and the polarizationconversion layer 24 shown in FIGS. 33(b) and 35(b), but is differenttherefrom in being located on the substrate 412 of the transmission typeliquid crystal display panel 610.

As described above, in this embodiment, a lightguide element 20B of theillumination device 620 includes the substrate 412, the polarizationselection layer 22 and the polarization conversion layer 24. Thelightguide element 20B also acts as a substrate of the transmission typeliquid crystal display panel 410.

The illumination device 620 in this embodiment can be produced, forexample, as follows.

First, as shown in FIG. 42(a), prisms are formed of a transparent resin29 having a refractive index of 1.53 on a phase plate 28 formed ofZEONOR (registered trademark) having a refractive index of 1.53, andthus a prism sheet 25″ having a thickness of 0.2 mm is formed. The prismsheet 25″ has a main surface (front surface) 25 a″ having asawtooth-like cross section and a generally flat rear surface 25 b″. Themain surface 25 a″ has inclining surfaces (inclining areas) 25 a 1″inclining with respect to the rear surface 25 b″, vertical surfaces(vertical areas) 25 a 2″ which are generally vertical to the rearsurface 25 b″, and parallel surfaces (parallel areas) 25 a 3″ which aregenerally parallel to the rear surface 25 b″. The inclining surfaces 25a 1″ are arranged increasingly densely as becoming farther from one endand closer to the other end of the prism sheet 25″ (as becoming fartherfrom the side surface which will act as the incidence surface 20 alater).

Next, as shown in FIG. 42(b), TiO₂ having a refractive index of 2.3 isvapor-deposited on the inclining surfaces 25 a 1″ of the main surface 25a″ of the prism sheet 25″ to a thickness of 65 nm, thereby formingdielectric films (dielectric thin films) 22 a. At this stage, dielectricfilms 22 b are also formed on the parallel surfaces 25 a 3″ of the mainsurface 25″.

Then, as shown in FIG. 42(c), the main surface 25 a″ of the prism sheet25″ is flattened by the transparent resin 29 having a refractive indexof 1.53, and the rear surface 25 b″ of the prism sheet 25″ is bonded tothe substrate 412 of the transmission type liquid crystal display panel610.

After that, the light source (for example, a cathode ray tube) 10 islocated on the incidence surface 20 a side of the lightguide element20B, and a reflection member (for example, a reflection film) 12 islocated so as to surround the light source 10. Thus, the illuminationdevice 620 shown in FIG. 41 is completed.

In the transmission type liquid crystal display 600 in this embodiment,the lightguide element 20B of the illumination device 620 as a backlightalso acts as a substrate of the transmission type liquid crystal displaypanel 610, and thus the illumination device 620 and the transmissiontype liquid crystal display panel 610 are integrated together.Therefore, the display apparatus is reduced in thickness.

When a difference refractive index layer having a different refractiveindex from that of the substrate 412 is provided between the substrate412 and the liquid crystal layer 413 of the transmission type liquidcrystal display panel 610, light incident on the inside of thelightguide element 20B from the light source 10 is reflected by theinterface between the substrate 412 and the different refractive indexlayer and thus is efficiently propagated in the lightguide element 20B.Therefore, the light from the light source 10 can be effectivelyutilized as illumination light.

When a polarizer is provided between the substrate 412 and the liquidcrystal layer 413 of the transmission type liquid crystal display panel610, the polarization direction of the light incident on the liquidcrystal layer 413 can be further uniformized, and thus the displayquality can be improved.

In this embodiment, the transmission type liquid crystal display 600having the illumination device 620 as a backlight integrated to thetransmission type liquid crystal display panel 610 is described. Thepresent invention is not limited to this, and is also preferablyapplicable to a reflection type liquid crystal display having anillumination device as a front light integrated to a reflection typeliquid crystal display panel.

INDUSTRIAL APPLICABILITY

According to the present invention, an illumination device capable ofcausing light from a light source to go out as light of a specificpolarization direction sufficiently efficiently is provided. When thisillumination device is used, an image display apparatus providing a highlight utilization efficiency and realizing bright display is provided.

An illumination device according to the present invention is preferablyusable especially as a backlight or a front light of a liquid crystaldisplay.

1. An illumination device, comprising: a light source; and a lightguide element including an incidence surface for receiving light emitted from the light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; and the polarization selection layer reflects the light of the specific polarization direction substantially only toward the outgoing surface.
 2. The illumination device of claim 1, wherein the polarization selection layer includes a plurality of inclining dielectric films provided at a predetermined angle with respect to the outgoing surface.
 3. An illumination device, comprising: a light source; and a lightguide element including an incidence surface for receiving light emitted from the light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; and the polarization selection layer includes a plurality of inclining dielectric films inclining with respect to the outgoing surface, and the plurality of inclining dielectric films are arranged increasingly densely as becoming farther from the incidence surface.
 4. The illumination device of claim 3, wherein: the lightguide element includes a first member having a main surface which includes a plurality of inclining surfaces inclining with respect to the outgoing surface and a plurality of parallel surfaces generally parallel to the outgoing surface, and a second member provided on the main surface of the first member for flattening the main surface; the plurality of inclining dielectric films are respectively formed on the plurality of inclining surfaces of the main surface; and the plurality of parallel surfaces of the main surface are arranged increasingly sparsely as becoming farther from the incidence surface.
 5. The illumination device of claim 4, wherein the polarization selection layer includes a plurality of further dielectric films respectively formed on the plurality of parallel surfaces of the main surface.
 6. The illumination device of claim 5, wherein the polarization selection layer is located in the vicinity of the outgoing surface and closer to the outgoing surface than the polarization conversion layer.
 7. The illumination device of claim 6, wherein the plurality of parallel surfaces are located closer to the outgoing surface than the plurality of inclining surfaces.
 8. The illumination device of claim 5, wherein the lightguide element further includes a counter surface facing the outgoing surface, and the polarization selection layer is located in the vicinity of the counter surface and closer to the counter surface than the polarization conversion layer.
 9. The illumination device of claim 8, wherein the plurality of parallel surfaces are located closer to the counter surface than the plurality of inclining surfaces.
 10. The illumination device of claim 4, wherein the first member is a prism sheet including a plurality of prisms arranged on the main surface.
 11. The illumination device of claim 4, wherein the second member is a transparent resin layer formed of a transparent resin material.
 12. The illumination device of claim 1, wherein the polarization conversion layer is formed of a transparent material having birefringence.
 13. The illumination device of claim 12, wherein the polarization conversion layer is an injection-molded transparent resin layer.
 14. The illumination device of claim 12, wherein the polarization conversion layer is a phase plate.
 15. The illumination device of claim 14, wherein directions of a slow axis and a fast axis of the phase plate in a plane parallel to the outgoing surface do not match the specific polarization direction.
 16. An illumination device, comprising: a light source; and a lightguide element including an incidence surface for receiving light emitted from the light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; and the polarization conversion layer is an injection-molded transparent resin layer having birefringence.
 17. An illumination device, comprising: a light source; and a lightguide element including an incidence surface for receiving light emitted from the light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; the polarization conversion layer is a phase plate; and directions of a slow axis and a fast axis of the phase plate in a plane parallel to the outgoing surface do not match the specific polarization direction.
 18. The illumination device of claim 15, wherein the phase plate has monoaxial refractive index anisotropy.
 19. The illumination device of claim 18, wherein a refractive index n_(x) in the direction of the slow axis of the phase plate, a refractive index n_(y) in the direction of the fast axis of the phase plate, a refractive index n_(z) in a thickness direction of the phase plate, a thickness d of the phase plate, a wavelength λ of visible light, and an angle α made by the specific polarization direction and the slow axis of the phase plate fulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈0, 0<(n_(x)−n_(y))·d<λ, and 10°<α<30° or 40°<α<60°.
 20. The illumination device of claim 18, wherein a refractive index n_(x) in the direction of the slow axis of the phase plate, a refractive index n_(y) in the direction of the fast axis of the phase plate, a refractive index n_(z) in a thickness direction of the phase plate, a thickness d of the phase plate, a wavelength λ of visible light, and an angle α made by the specific polarization direction and the slow axis of the phase plate fulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈0, (n_(x)−n_(y))·d=λ/2, and 10°<α<30°.
 21. The illumination device of claim 18, wherein a refractive index n_(x) in the direction of the slow axis of the phase plate, a refractive index n_(y) in the direction of the fast axis of the phase plate, a refractive index n_(z) in a thickness direction of the phase plate, a thickness d of the phase plate, a wavelength λ of visible light, and an angle α made by the specific polarization direction and the slow axis of the phase plate fulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈1, λ/4<(n_(x)−n_(y))·d<5λ/4, and 20°<α<90°.
 22. The illumination device of claim 18, wherein a refractive index n_(x) in the direction of the slow axis of the phase plate, a refractive index n_(y) in the direction of the fast axis of the phase plate, a refractive index n_(z) in a thickness direction of the phase plate, a thickness d of the phase plate, a wavelength λ of visible light, and an angle α made by the specific polarization direction and the slow axis of the phase plate fulfill the relationship of (n_(x)−n_(z))/(n_(x)−n_(y))≈1, (n_(x)−n_(y))·d=λ/2, and 20°<α<80°.
 23. The illumination device of claim 15, wherein the phase plate has biaxial refractive index anisotropy.
 24. The illumination device of claim 23, wherein a refractive index n_(x) in the direction of the slow axis of the phase plate, a refractive index n_(y) in the direction of the fast axis of the phase plate, a refractive index n_(z) in a thickness direction of the phase plate, a thickness d of the phase plate, a wavelength λ of visible light, and an angle α made by the specific polarization direction and the slow axis of the phase plate fulfill the relationship of 0.6<(n_(x)−n_(z))/(n_(x)−n_(y))<0.9, λ/4<(n_(x)−n_(y))·d<3λ/4, and 60°<α<80°.
 25. The illumination device of claim 23, wherein a refractive index n_(x) in the direction of the slow axis of the phase plate, a refractive index n_(y) in the direction of the fast axis of the phase plate, a refractive index n_(z) in a thickness direction of the phase plate, a thickness d of the phase plate, a wavelength λ of visible light, and an angle α made by the specific polarization direction and the slow axis of the phase plate fulfill the relationship of 0.6<(n_(x)−n_(z))/(n_(x)−n_(y))<0.9, (n_(x)−n_(y))·d=λ/2, and 60°<α<80°.
 26. The illumination device of claim 1, wherein the polarization conversion layer is located oppositely to the outgoing surface with the polarization selection layer interposed therebetween.
 27. The illumination device of claim 1, wherein the polarization conversion layer is located closer to the outgoing surface than the polarization selection layer.
 28. An image display apparatus, comprising: the illumination device of claim 1; and a display panel provided on the outgoing surface side of the lightguide element of the illumination device and including at least one polarizer.
 29. The image display apparatus of claim 28, wherein the illumination device further includes a transparent input device formed on the counter surface of the lightguide element.
 30. The image display apparatus of claim 29, wherein: the display panel includes a substrate; and the lightguide element included in the illumination device acts as the substrate.
 31. A lightguide element including an incidence surface for receiving light emitted from a light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element further includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; and the polarization selection layer reflects the light of the specific polarization direction substantially only toward the outgoing surface.
 32. A lightguide element including an incidence surface for receiving light emitted from a light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element further includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; and the polarization selection layer includes a plurality of inclining dielectric films inclining with respect to the outgoing surface, and the plurality of inclining dielectric films are arranged increasingly densely as becoming farther from the incidence surface.
 33. A lightguide element including an incidence surface for receiving light emitted from a light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element further includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; and the polarization conversion layer is an injection-molded transparent resin layer having birefringence.
 34. A lightguide element including an incidence surface for receiving light emitted from a light source and an outgoing surface from which the light incident from the incidence surface goes out; wherein: the lightguide element further includes a polarization selection layer for causing light of a specific polarization direction, among the light incident from the incidence surface, to selectively go out from the outgoing surface, and a polarization conversion layer for converting light of a polarization direction, different from the specific polarization direction, into the light of the specific polarization direction; the polarization conversion layer is a phase plate; and directions of a slow axis and a fast axis of the phase plate in a plane parallel to the outgoing surface do not match the specific polarization direction. 